Our app recently got rejected couple times by an issue that the keyboard is not appeared (or is disappeared after 1 second) on log in page when the reviewer tried to login by using their iOS 15.0.1 device (iPhone or iPad).
We can't reproduce from our devices until we connect a Magic Keyboard to our device and found out the iOS 15 has been changed for the behaviour of hardware keyboard:
When running the app in iOS 15 and iOS 13 in Simulator, if the "Connect Hardware Keyboard" is on, typing in hardware keyboard results differently for iOS 15 and iOS 13, iOS 15 keyboard is disappeared from the Simulator.
I connect my Magic Keyboard to my iPhone (iOS 15.0.1) and when I tap on a text field (e.g. in Message app) - there is no iOS keyboard appears on the screen.
I also sometimes have luck that the iOS keyboard is appeared when I tap on Message app's text field, if the iPhone is awake from the locked screen. Then I can use the iOS keyboard for inputting, but once I start typing in the Magic Keyboard, the iOS keyboard is disappeared at once.
It seems there is a bug for iOS 15 when a hardware keyboard is connected to the iPhone.
iOS Keyboard showing (for the first time only, or awake from locked screen):
iOS keyboard disappeared when typing from the hardware keyboard:
iOS 13 has no such issue: