Outlook add-in not working via outlook client

Question

Hi,
Our company has our own XML manifest button. [custom add-in].
We have support for shared folders tag to allow shared mailboxes to use this button too.
But for some reason, specific customers are having issues using this button via outlook client [WORKS ON OWA]
the error message they are getting: "API not supported for shared folders"

Additional details :
The tag <SupportsSharedFolders>true</SupportsSharedFolders> is in place in the XML.
The client version is built 2210/2209 which supports the requirement set of 1.8 [which introduced shared folders support]
Everything works perfectly on outlook web application the error occurs only via outlook client

Any suggestion why this could happen?

Here is how our XML manifest looks like:

<OfficeApp  
        xmlns="http://schemas.microsoft.com/office/appforoffice/1.1"  
        xmlns:bt="http://schemas.microsoft.com/office/officeappbasictypes/1.0"  
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  
        xsi:type="MailApp">  
    <Id>XXXX-XXXX-XXXX-XXXX</Id>  
    <Version>1.0.0</Version>  
    <ProviderName>XXXXX</ProviderName>  
    <DefaultLocale>en-US</DefaultLocale>  
    <DisplayName DefaultValue="XXXX"/>  
    <Description DefaultValue="XXXX"/>  
    <IconUrl DefaultValue="link to logo"/>  
    <HighResolutionIconUrl DefaultValue="link to logo"/>  
    <Hosts>  
        <Host Name="Mailbox"/>  
    </Hosts>  
    <Requirements>  
        <Sets>  
            <Set Name="MailBox" MinVersion="1.1"/>  
        </Sets>  
    </Requirements>  
    <FormSettings>  
        <Form xsi:type="ItemRead">  
            <DesktopSettings>  
                <SourceLocation DefaultValue="our endpoint"/>  
                <RequestedHeight>100</RequestedHeight>  
            </DesktopSettings>  
            <TabletSettings>  
                <SourceLocation DefaultValue="our endpoint"/>  
                <RequestedHeight>100</RequestedHeight>  
            </TabletSettings>  
            <PhoneSettings>  
                <SourceLocation DefaultValue="our endpoint"/>  
            </PhoneSettings>  
        </Form>  
    </FormSettings>  
    <Permissions>ReadWriteMailbox</Permissions>  
    <Rule xsi:type="RuleCollection" Mode="Or">  
        <Rule xsi:type="ItemIs" ItemType="Message" FormType="Read" IncludeSubClasses="true"/>  
    </Rule>  
    <DisableEntityHighlighting>false</DisableEntityHighlighting>  
    <VersionOverrides  
            xmlns="http://schemas.microsoft.com/office/mailappversionoverrides"  
            xsi:type="VersionOverridesV1_0">  
        <Hosts>  
            <Host xsi:type="MailHost">  
                <DesktopFormFactor>  
                    <FunctionFile resid="function-file-url"/>  
                    <ExtensionPoint xsi:type="MessageReadCommandSurface">  
                        <OfficeTab id="TabDefault">  
                            <Group id="our-control-group">  
                                <Label resid="our-label"/>  
                                <Control xsi:type="Button" id="report-button">  
                                    <Label resid="report-button-label"/>  
                                    <Supertip>  
                                        <Title resid="report-button-label"/>  
                                        <Description resid="report-button-description"/>  
                                    </Supertip>  
                                    <Icon>  
                                        <bt:Image size="16" resid="logo-16"/>  
                                        <bt:Image size="32" resid="logo-32"/>  
                                        <bt:Image size="80" resid="logo-80"/>  
                                    </Icon>  
                                    <Action xsi:type="ShowTaskpane">  
                                        <SourceLocation resid="report-task-pane-url"/>  
                                    </Action>  
                                </Control>  
                            </Group>  
                        </OfficeTab>  
                    </ExtensionPoint>  
                </DesktopFormFactor>  
            </Host>  
        </Hosts>  
        <Resources>  
            <bt:Images>  
                <bt:Image id="logo-16" DefaultValue="our logo"/>  
                <bt:Image id="logo-32" DefaultValue="our logo"/>  
                <bt:Image id="logo-64" DefaultValue="our logo"/>  
                <bt:Image id="logo-80" DefaultValue="our logo"/>  
            </bt:Images>  
            <bt:Urls>  
                <bt:Url id="report-task-pane-url" DefaultValue="our endpoint"/>  
            </bt:Urls>  
            <bt:ShortStrings>  
                <bt:String id="report-button-label" DefaultValue="XXXX"/>  
                <bt:String id="our-label" DefaultValue="XXXX"/>  
            </bt:ShortStrings>  
            <bt:LongStrings>  
                <bt:String id="report-button-description"  
                           DefaultValue="our description"/>  
            </bt:LongStrings>  
        </Resources>  
        <VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides/1.1"  
                          xsi:type="VersionOverridesV1_1">  
            <Hosts>  
                <Host xsi:type="MailHost">  
                    <DesktopFormFactor>  
                        <SupportsSharedFolders>true</SupportsSharedFolders>  
                        <FunctionFile resid="function-file-url"/>  
                        <ExtensionPoint xsi:type="MessageReadCommandSurface">  
                            <OfficeTab id="TabDefault">  
                                <Group id="our-control-group">  
                                    <Label resid="our-label"/>  
                                    <Control xsi:type="Button" id="report-button">  
                                        <Label resid="report-button-label"/>  
                                        <Supertip>  
                                            <Title resid="report-button-label"/>  
                                            <Description resid="report-button-description"/>  
                                        </Supertip>  
                                        <Icon>  
                                            <bt:Image size="16" resid="logo-16"/>  
                                            <bt:Image size="32" resid="logo-32"/>  
                                            <bt:Image size="80" resid="logo-80"/>  
                                        </Icon>  
                                        <Action xsi:type="ShowTaskpane">  
                                            <SourceLocation resid="report-task-pane-url"/>  
                                        </Action>  
                                    </Control>  
                                </Group>  
                            </OfficeTab>  
                        </ExtensionPoint>  
                    </DesktopFormFactor>  
                    <MobileFormFactor>  
                        <FunctionFile resid="function-file-url"/>  
                        <ExtensionPoint xsi:type="MobileMessageReadCommandSurface">  
                            <Group id="mobilegroupid">  
                                <Label resid="our-label"/>  
                                <Control xsi:type="MobileButton" id="report-button">  
                                    <Label resid="report-button-label"/>  
                                    <Icon xsi:type="bt:MobileIconList">  
                                        <bt:Image size="25" scale="1" resid="logo-32"/>  
                                        <bt:Image size="25" scale="2" resid="logo-32"/>  
                                        <bt:Image size="25" scale="3" resid="logo-32"/>  
  
                                        <bt:Image size="32" scale="1" resid="logo-32"/>  
                                        <bt:Image size="32" scale="2" resid="logo-32"/>  
                                        <bt:Image size="32" scale="3" resid="logo-32"/>  
  
                                        <bt:Image size="48" scale="1" resid="logo-32"/>  
                                        <bt:Image size="48" scale="2" resid="logo-32"/>  
                                        <bt:Image size="48" scale="3" resid="logo-32"/>  
                                    </Icon>  
                                    <Action xsi:type="ShowTaskpane">  
                                        <SourceLocation resid="report-task-pane-url"/>  
                                    </Action>  
                                </Control>  
                            </Group>  
                        </ExtensionPoint>  
                    </MobileFormFactor>  
                </Host>  
            </Hosts>  
            <Resources>  
                <bt:Images>  
                    <bt:Image id="logo-16" DefaultValue="our logo"/>  
                    <bt:Image id="logo-32" DefaultValue="our logo"/>  
                    <bt:Image id="logo-64" DefaultValue="our logo"/>  
                    <bt:Image id="logo-80" DefaultValue="our logo"/>  
                </bt:Images>  
                <bt:Urls>  
                    <bt:Url id="report-task-pane-url" DefaultValue="our endpoint"/>  
                </bt:Urls>  
                <bt:ShortStrings>  
                    <bt:String id="report-button-label" DefaultValue="XXXX"/>  
                    <bt:String id="our-label" DefaultValue="XXXX"/>  
                </bt:ShortStrings>  
                <bt:LongStrings>  
                    <bt:String id="report-button-description"  
                               DefaultValue="our description"/>  
                </bt:LongStrings>  
            </Resources>  
        </VersionOverrides>  
    </VersionOverrides>  
</OfficeApp>