Looks like maybe the app thinks the screen is slightly narrower than it is. Reporting from an iPhone 15 Pro on iOS 17.2.1.
Repro steps:
  1. Swipe over to controls/player view
  2. Quit Longplay
  3. Open Longplay by tapping on the Now Playing widget (not on a button, just on the edge of the widget)
  4. There is a black bar on the right edge of the screen. Swiping back and forth between views removes it.
Hopefully this is helpful!