Hi,
I'm on a pixel 4xl running Android 11. I was having the same problem. I could play a song in Poweramp by selecting "open with" from Google Files but it would never remain in the Poweramp library and Poweramp library wouldn't scan or find any of my mp3s.
I think the way I fixed it was by going into settings in the Poweramp app (click the three horizontal bars in bottom right then click settings) then scroll down to "Misc" then scroll down to "File access legacy mode" and turn it off. It restarts the app and after that I was able to scan and library populated automatically.
Hope it helps.