I have the excact same problem!
I recently upgraded from a Fairphone 2 (Android 7) to a Fairphone 3 (Android 9), transferred the user data via Google and put my old SD card into the new phone. Everthing seemed fine at first with my whole Poweramp library being transferred. But having just saved some new tracks on my SD card I realized that the tag info was missing. I tried to insert the tags with Poweramp app then - but they weren't saved.
After having discovered this thread I moved those songs to the internal storage and they are being read fine. Moving them back to the SD card has the tag info missing again.
I also moved an older track that was shown correctly to internal storage and back to SD - and this also results in missing tag infos (having rescanned after each swap).
So, I'm gonna store my new songs in the internal storage now - but I hope you can fix this problem soon, so all my music can be back on the SD card!
EDIT: Oh, and by the way... Google's "Play Music" app is able to read the tags from the SD card just fine.
EDIT 2: I've finally got it working. I realized that Poweramp does recognize the tag info, when I transfer the files from my compuer to the phone's SD card via USB cable - and not Wifi as I did before. No idea why the tag data gets lost via Wifi (but only on SD and only in Poweramp not in Google Play Music...), but... hey... I've found a workaround to store my new files on the SD card again!