Think I found my issue ...
... I had previously disabled Documents OnePlus. Re-enabling this seems to allow me to select the storage.
(if someone else has this problem and disabled or debloated Documents - try re-enabling it)
I have the same problem OnePlus 6 Android 10.
I've tried ...
* manually allowing storage permission
* clearing the app data
* uninstalling/reinstalling Poweramp - still can't propagate the music library.
Other music players working fine.