I have absolutely no idea if this will help anyone but I’m going to post it just in case.
Since the update to iPadOS 13.4 I have been struggling with my third-party mouse and scrolling. As a disabled user who also uses AssistiveTouch and a few other things this is a big problem for me, although that doesn’t seem to be relevant here as my troubleshooting plus all of your experiences seem to suggest that this is an issue with a pointer Control in general. I have just told my iPad to completely forget my Bluetooth mouse and then I reconnected it and suddenly scrolling is working again. I have no idea if this is a permanent fix as I have had periods of it working and then suddenly periods of not. I’m sure some of you have already tried this, but I just thought I would post it and see if it helps anyone.