I went there yesterday (Single-player mode) and had this happen with all bosses.
More precisely, when I reached the first boss (the big cat) there were two of him in the clearing - one alive, one already dead (I believe I just found Schrödinger’s cat!). All doors were open, including the magical barrier and the stone door towards the entrance/exit area. As soon as I killed the cat, all doors slammed shut - the magical barriers blocked by way back to where I came from as well as the way forward, and the stone door the shortcut to the exit.
I removed my bracelet and respawned at the dungeon entrance, and the shortcut to the cat room was open. I went through, found the big cat alive again, let it live and sneaked past, proceeded onwards. Then I wasn’t sure if I could actually proceed if the cat was alive, turned back, killed the cat again and found myself trapped exactly as before. So I got bored, activated ghost mode in admin panel and went through the barrier. Then ghost mode off.
I reached the gorilla clearing. Saw that the gorilla was both dead and alive (damn you, Schrödinger!) and all doors open. Left the living gorilla alive and just ran past him towards the next area.
Reached the bull. This one was just alive, there was no extra bull corpse that I could see (looks like Schrödinger didn’t get this far). The mists were blocking the exit from sight so I couldn’t tell if it was open or closed. But, encouraged by the fact that there was no dead evil twin of the bull present I boldly leaped in, stabbed him to death and watched the mists disperse… and reveal a closed stone door. Bah. Ghost moded myself through.
Final boss. Again, a dead clone had joined the living brother (nope, Schrödinger had just skipped the bull). Some time later there were two dead brothers (praised be my Captain who face-tanked the werehyena like a pro while I perforated it with snake arrows). Jhebbal Sag peeked over the fence, I harvested the boss and ate him to unlock the altar’s recipe. And - you guessed it, the way out of the dungeon was blocked by a now all too familiar looking stone door. Ghost moded through and exited the dungeon.
So yeah, it does look like the triggers to open/close the doors and barriers have been reversed, making the dungeon a little bit more frustrating than usual.
My Single-player game does use some mods, but I’ve been in the Midnight Grove before, with the same mod setup, and it has worked fine. Nevertheless, I was ready to blame some modder for this until I saw this thread. If it happens on officials too, then I guess the problem is within the Grove itself.