For me this kind of confirms that some kind of library was modified in one of these last post-patch updates, which is not reset when reinstalling patch 7.
Damn, I'm sorry Ido!
Only thing I can think of is if you have tried downloading a specific Patch 7 version from Steam Archives. Would not that version have the old dll's?
I'm not sure if you are doing the old branch from the Beta tab when you say going back to Patch 7. And if you are, I would suggest trying a fresh download from an old version.
Doing it this way you have to go through Steam console and you have to know the number of the version you want. AND you can not interrupt the download or it will abort the whole download since it's downloading through Steam console.
A Guide:
Rollback BG3 Steam
