@Autumn Fox Use the XML like below and you are good to go. So the reason here is every app has a unique AppID for New Teams it is MSTeams_8wekyb3d8bbwe!MSTeams
<?xml version="1.0" encoding="utf-8"?>
<LayoutModificationTemplate
xmlns="http://schemas.microsoft.com/Start/2014/LayoutModification"
xmlns:defaultlayout="http://schemas.microsoft.com/Start/2014/FullDefaultLayout"
xmlns:start="http://schemas.microsoft.com/Start/2014/StartLayout"
xmlns:taskbar="http://schemas.microsoft.com/Start/2014/TaskbarLayout"
Version="1">
<CustomTaskbarLayoutCollection PinListPlacement="Replace">
<defaultlayout:TaskbarLayout>
<taskbar:TaskbarPinList>
<taskbar:UWA AppUserModelID="MSTeams_8wekyb3d8bbwe!MSTeams" />
</taskbar:TaskbarPinList>
</defaultlayout:TaskbarLayout>
</CustomTaskbarLayoutCollection>
</LayoutModificationTemplate>