Mine has worked since Whonix v13 (and probably earlier IIRC) if I simply do the steps @tatertot mentions above. It seems pretty standard for the Tor Browser here.
Only now with this update to 8.5 has it suddenly failed, and much worse, deleted my entire existing password store. (It’s a good thing I do backups.)
Anyway, turns out it is an upstream tor browser bug, now acknowledged and with a simple fix to restore passwords back easily.
So, problem solved solvable.