I believe I’ve solved this, recording here for anyone that runs into this issue!
For some reason, the fact that I allowed new users to login with either their username or email seems to be the issue. I unchecked this setting in System Console, then created an account with a NEW email address, and was able to login with this new account on Android and iOS. The previous account made when the setting was not changed was still unable to login via app.
When I tried to delete my old account that wasn’t able to login, I was unable to. I tried using the mmctrl command line tool, but got a SQL error about the user ID not being found.
Not sure exactly how it got messed up, but that’s what worked for me!