The only persistent issue on my wife’s 12.9 original iPad Pro is that the screen frequently rotates 180 degrees while she’s looking at it. She, as I do as well, always has it in landscape view. I don’t know if it would be as frequent in portrait but it is annoying. She’s currently on 13.3. I had the same issue initially on my 3rd gen 12.9 but not since the early 13 updates.