Switching the music app off doesn’t make any difference for me. Audiobooks are the worst thing affected, non-stop stuttering if I have any other app open in the foreground other than Books or when I open any other app - which makes me wonder if it’s a matter of the OS not being able to continually buffer audio in advance properly any more. It’s the kind of thing you get using a DAW when your audio driver buffer isn’t being filled fast enough so it keeps emptying. Usually with DAWs it’s a sign of an over-worked cpu that can’t keep up with the demand on it or storage isn’t being accessed quickly enough.
IPad audio used to be one of the thing’s strongest assets, but somehow Apple seem to have managed to break it, just like the early Mojave releases and the introduction of the T2 chip caused chaos for people recording or producing audio or video because somehow the USB system wasn’t working correctly with USB or Thunderbolt attached professional audio interfaces any more. Apple fixed the problem (pretty much) after a few months but it was a real nuisance when it was happening and caused real problems for people who use Macs for what has always been their strongest feature, Core Audio.