Had this same issue occur multiple times. Unless they can fix the bug where enemies fall through the map/out the walls there's not much to be said about it. I mentioned the issue in QOL changes when stating that they should add a function to cancel quests that are not completable due to this issue.
Ideally they will fix it, but in the meantime we are losing out on items, quest rewards, and potentially maps etc. In certain instances you can't even leave the quest at all when it happens.
The only thing that can be done is to exit the game to main menu and reload, losing the dungeon in the process. It's pretty sad, and it doesn't seem like it would be that hard to enable some code that prevents bosses/large mobs from being spawned within a certain distance of walls. Hopefully they fix this issue asap