We have this issue with one costumer also. But seems to only occur in their tenant, for both existing and new users. If we log in with users from other tenants on the same machine everything works.
We have tested this on no less than 5 different MacBooks with both Catalina as well as Big Sur betas.
Always reproducible.
Log in with a user from the affected tenant, autosave never works
Log in with a user from a different tenant, autosave always works.
I've put more than 50 hours on troubleshooting this with reinstalls, testing different versions, different computers as well as working with the support.