Apple Music opens automatically when pressing the physical Play/Pause button
I have a FiiO JM21, and whenever I press the physical Play/Pause button, Apple Music launches automatically. I first noticed this behavior when connecting the JM21 to an external DAC via USB. Even when using Pure Music mode, which launches the FiiO Music app, pressing the physical Play/Pause button causes Apple Music to open and start playing music at the same time as FiiO Music. When swiping down to access the Android quick settings menu, you can see both the FiiO Music and Apple Music mini-players (by swiping right on the mini-player). Essentially, both music apps run simultaneously after pressing the physical "play/pause" button.
I've already tried disabling Picture-in-Picture by going to Settings > Apps > Apple Music > Advanced, but it didn't help. I also changed the battery setting from Unrestricted to Optimized, but that didn't solve the problem either.
At this point, I have no idea why Apple Music is taking exclusive control of the physical media buttons.