First, let me say that I'm reporting this right now only because people running beta versions are supposed to report things.
I've recently been encountering the case that when some other application opens a folder, the folder opens in Explorer rather than Opus.
I first saw this after installing 12.9.3, but the timing made me suspect that possibly Microsoft's September "Patch Tuesday" updates to Windows 10 could possibly be relevant since simply reinstalling 12.9.2 didn't fix the problem.
I won't try to list everything I've done to try to isolate the circumstances of the problem. (I don't even know anymore)!
Suffice it to say that at the moment I have a system with the "Patch Tuesday" updates and 12.9.2 installed which seems to probably not have the issue, but the problem was unpredictable enough, or at least seemed so to me, that I feel I have to run this configuration for a while before I'm confident that it's stable and doesn't have the problem.
I did just copy the system that seems to now be OK and installed 12.9.3 on it without immediately encountering the problem, but I don't consider that conclusive because I had other cases where I thought I had solved the problem only to have it come back.
My experience so far has convinced me that I have to use the system with 12.9.2 installed for a while without seeing the problem before I can be convinced that if I install 12.9.3 again and the problem comes back that will really be because of the installation of 12.9.3.
I realize this can't be a common problem with 12.9.3 and am even mentioning in now only so I won't be told "Why didn't you report this when you were running the relevant beta".
I'm completely happy to have no response to this unless I come back later and say I've encountered the problem again.