Same. I updated OP in a comment thread above with my experiments for my case. Mine triggered after the crossroads boss behind the three-revenant seal without major story progress (took the mandatory companion quests from Lucanis and Davrin and the rest of the optional companion quests) and i cleaned up all side quests and possible collectibles in case the bug came back. I reloaded a save right before the encounter but AFTER the gates opening and im keeping that boss for very last so as to not block off collectibles in case the last two acts dont update the world again.
though understanding the 3 main acts’ transition points from quest lists online seems to be that “world state update” i was theorizing. Since our “fix” happened after act 2 started (after weisshaupt). Im extra vigilant now. I leave a couple easily accessible altars to verify if i got bugged again. Thankfully i found out right away after the boss since the chest wouldnt open and thankfully the reloaded save after all the gates work unless id backtrack a few side quests before that from a manual save.
i speculated initially that it could be the gates or some ability interactions. Now im also adding the potential for certain enemies or enemy types which was still debunked in my experience since i also just beat a similar big hp boss in necropolis without issues. So if its specific bosses or certain conditions for them im uncertain at this point. Im also adding my thoughts that it could be an input issue when battles get tense and i mash buttons too much with panic mode on 🤣, but i tried to beat the boss a second time doing my best not to mash or spam abilities and still the interactables locked out in either case potentially debunking the input error stuff. 🤔 so as for the cause, i still have no definitive evidence, but all i know is i know what triggered it the second time for me. It was the boss in the Crossroads called….
MINI SPOILERS
The Revenant Dragon.