EDIT: Problem solved 🙌 (check out answer below)
I got bluetooth headphones almost two years ago and Poweramp eight years ago, and have had no problem with using the headphones with the app before.
But a couple of weeks (maybe a month) ago, it started to not work properly. The sound and everything else works great, but if I pause Poweramp using the play/pause button on the headphones, I can’t start it up again with the button. Poweramp now also disappears from the lock screen when I pause, so I then have to unlock the screen, and open up Poweramp to start again where I paused.