I have two apps on the new Maker plan. Both apps are not allowing certain users to log in - I suspect it is only custom gmail domains that are having the issue as this is the only way I have been able to replicate it. Privacy settings are public with required sign in and other users are managing to sign in.
Having the same issue with two different apps - my only two apps on the new Maker plan.
I just changed a different app to the new Maker plan from a Legacy pro plan to test whether the plan was significant. Immediately before changing the plan I could log in with the custom domain email address. Sure enough, changing it to Maker plan means I can no longer log in with that address. So there is something up with the way users are being authenticated on the new plan.
One of the apps is a Classic app and the other is a new app so the type of app is not significant.
Screen recording: Loom | Free Screen & Video Recording Software | Loom
Any ideas?