I have a similar issue. I use Neutron Music Player and it worked fine with IOS for years.
With IOS17 it will not autoplay with connected bluetooth devices like it did before. I need to unlock the screen to be able to start music. Sometimes music stops after a track when on locked screen, but this behavior is not consistent.
It is quite clear though that the problem is related to the lock screen. In my car it is even worse. I can only start music once I have unlocked my phone, opened the app and pressed PLAY PAUSE PLAY. It would not be controllable through the HMI of the car (MBUX in that case). I can see that the phone is connected, the album art shows, but the music does not start before at least unlocked (or app opened and PLAY PAUSE PLAY sequence is performed, which is the only definite way to start music at all times).
Very frustrating problem that never happened in the past. Only with IOS 17. Please advice on steps for resolution. The phone is unusable for music with bluetooth right now. Thank you!