OneDrive app Sync with Azure App Proxy not working.

Question

Hi,

We have an on-premises SharePoint 2019 farm and we have configured an Azure Web App Proxy so users external to the corporate network can access the farm. This works fine, however it causes an issue if users attempt to sync their OneDrive files hosted on the 2019 farm using the OneDrive sync client.

We have the mysites\onedrives web app set up to use NTLM on the default zone (this is only used by search) and we have the app proxy acting as a trusted identity provider on the custom zone - This is being used by external users to authenticate using the Azure app proxy.

If we sync the OneDrive client to the URL on the default zone it works fine. If we try and sync the OneDrive using the custom zone and AZ app proxy we get an error of "There was a problem connecting to OneDrive 0x8004deef"

It seems as though the OneDrive client doesnt like being pointed at the AZ app proxy. Can anyone confirm if the OneDrive sync client is compatible with AZ App proxy and if so, what additional steps are needed to configure it to work?