iPad Air 1 - FaceTime app crashes after a period of time

My iPad Air 1 is running iOS 12.4.8 - when I FaceTime my sister, whose iPad is also on 12.4.8, we have no issues. When I FaceTime my mother who has an iPad Pro running iOS 14, the connection crashes after 5 or so minutes and I have to restart my iPad each time.

Is is there a known iOS compatibly issue between iOS 12.4.8 and iOS 14? If so, what is apple doing to rectify it? Other than this problem my iPad Air is running perfectly.

I have tried: deleting the app and reinstalling it; resetting to factory settings and reinstalling my profile; resetting to factory settings and setting up as new.

An extremely frustrating issue. Thanks in advance to anyone who can help!