We have the same troubles. We are three active administrators with full Organization access and TFA deactivated and all three have the same settings in Administrators overview.
Only one Admin is able to log in through the App, two of us are experiencing the same issues as already mentioned in this thread.
Tried on different devices with fresh installed Apps.
Two devices are an iPhone 7 32 GB iOS 12.2, another device iPhone 7 128 GB iOS 12.2
The admin with working log in runs on iPhone 7 32 GB iOS 11.3.1