The Teams app is requiring users to log in every time they open it.
- Teams Integration
When signing into a community with a vanity URL (companyname.onfirstup.io) via the Firstup tab app for Teams, clicking out of and back into the app will sign the user out.
Currently the only workaround is to avoid navigating away from the Firstup app, and sign back in every time it is selected.
Please check the current status of this issue on the Known Issues Tracker. (Issue ID: CSUP-3120)