No, I don't have File Access Legacy Mode enabled, and I've never touched that setting.
EDIT: I've tried turning it on, selecting my music folder, and this interestingly this actually duplicated every file, even with a Full Rescan... I think it was seeing every file through the legacy mode AND the new mode. Had to temporarily disable Legacy Mode in order to remove the new mode's access there. And upon rebooting, Poweramp complained for a second that it couldn't see the files, but a couple seconds later, everything was fine! So, yeah, Legacy Mode actually works fine for me through reboots.
The Poweramp app has all permissions granted already. "Remove permissions if app isn't used" is not enabled.
I've attempted removing all permissions (including "external storage" ones) then granting them again, but this did not solve the issue.
Through the log generation feature I see a bunch of errors identical to this one on rescan failure:
12-23 04:28:22.542 4325 7351 E dirscanner.c: native_readdir !fail errno=Operation not permitted
The fact that the issue can get fixed simply by tapping "music folders" then immediately backing out, without changing any settings, is quite odd to me... I've sent the full log via email.