email sent to support.
Martin,
Since no one as AM could figure out what this bug was I spent time today going over each setting one by one to figure it out, you can imagine how much time it took but it should have been easy enough for your developers to figure out in a sec if you saw my videos and noticed the message background!
It has nothing to do with Android 11 and it is strictly related to AM as I stated prior other email apps works just fine.
The bug is related to Settings>Composing and Sending > "New message Editor"
With this turned off, which is how I use it, because I don't want a white background for obvious reasons and AM could not figure out how to implement this feature with a black background, the keyboard does not pop up when replying or forwarding the message. This is repeatable 100% on multiple devices.
Now that you have the information, fix the bug finally since I had to do the leg work for your team.