Hi there.
I've just stumbled upon the very same problem.
As even yesterday updates on Beta stream fully worked for me, I don't think it's a matter of some update in particular being incompatible with Wine/Linux.
On the other hand, fresh copy installed from today's osu!install.exe locks-in too.
None of proposed solutions (changing the _ReleaseStream and launching with .require_update file or with -repair flag) works - updater starts and tells that osu~ is already up to date, then the game starts, and _ReleaseStream value is reset.
In osu!.cfg I see a lot of h_filename = hash pairs (15 lines).
As Jeebasaurus mentioned copying DLL-s from a working installation, this may or may not be related.
As for what triggered it for me - I changed renderer to "compatibility" mode, as sometimes osu! crashed randomly at beatmap selection screen (not really often), and I was wondering if this setting will help anything. Osu restarted immediately after checking the checkbox, downgraded to Stable (Fallback) and entered the lock-in.
Removing (renaming) the osu!name.cfg config file didn't help for the lock-in.
I have a backup from a few weeks ago of the whole wineprefix with osu! installed on laptop.
I will try to restore from it and tell if it helped.
peppy, would you be able to tell us about what triggers the "Stable (Fallback) lock-in" mechanism?
Aside from "intentionally restricted when running under operating systems which can't support it" - my system could support is yesterday, but for others the lock-in happened earlier, so it seems to be too random for "an incompatible update has been just released" kind of situation.
People having problem mentioned in this thread are probably technical (I am a programmer and using Linux for 3 years myself), so please, don't tell us things that you would tell to children who don't know how to really use their PC (Sorry, but "intentionally restricted" really does sound that bad).
Oh, and why can't I just happily use the fallback version? Well, for some reason performance drops to ~20FPS, welp.
Sorry for the long post. >_<