Apple Music app resizing and moving

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Magnet modifies how the system handles windows, so it is at least a very likely suspect.

Try in Safe Mode, as Old Toad suggested. Most likely, this will not happen in Safe Mode, indicating that a system modification may be involved. If that is the case, try temporarily uninstalling Magnet and see if the problem persists.