I just had a similar experience recently and while researching the issue it brought me to this post.
I originally had an S9 with a 128GB SD card that contained roughly around 80 gig of music. When I updated my phone to an S20, I transferred the card and noticed a few glitches. Not necessarily all involving Poweramp but also syncing in general. Not sure if the card became corrupted but I was no longer able to sync music ripped from CD's through the Windows Media Player. This method worked briefly after the switch but was very glitchy and got to the point it stopped working altogether. It wouldn't recognize the card but if it was removed it would recognize the phone.
I bought a new card and transferred all the info over to it, however I can't get the music to play with the Poweramp app. Everything is showing up including album art and playlists but when you hit play a message that says "Failed to play / Storage unmounted" appears. However the card was mounted properly and acces was granted to the folders.
Any help would be greatly appreciated, thanks.