It is not just Android and the Mantis having these types of issues. The iOS version of Breeze Cam for the Yuneec Breeze was broken in iOS 14. It will work for one connection to the aircraft, but subsequent attempts cause the app to crash back to the main screen. It has to be deleted and reinstalled to stop the app from crashing.
While Android is very flexible, it also has issues like PC’s running Windows, where running the OS on many different hardware platforms can make software development a nightmare. So Android 12 may be the culprit or the S21, it is hard to say.