Forms embedded in SharePoint doesn't display in custom Teams app

Dinesh-MSFT Thank you for your explanation and research, but requiring server side components for embedding SharePoint in Teams is not a good solution. This stopped working some time this winter, and I assume it's caused by security hardening in Teams.

SharePoint is the man in the middle here and should be allowed to request obo tokens when embedded in Teams.

All parties involved here are Microsoft first party apps; Teams, SharePoint and Forms. There's no third party code involved in this custom Viva Connections setup, just the teams app manifest/json.

Is it correct that it's SharePoint that enforces the CORS restrictions on the OBO API endpoint?