Hi everyone,
I had similar problem after migrating from Huawei Mate 9 to OnePlus 8T. I was desperate after days of trying. My mistake was ... I always started Poweramp using the Poweramp symbol. Once I started Poweramp through Poweramp Full Version Unlocker app ... and I was amazed. I was able to select the music folder, rescan it and everything works perfectly for me now :)
Hopefully the tip will help someone.
Kirpan