Strongbox won't open after spawning some of the monsters - Mire map layout

This is the second time, on this map layout, I have had a strongbox only spawn a few of the mobs, then stay locked and not spawn the other monsters:

https://images.steamusercontent.com/ugc/33319299274987996/F91265DD7309BE81D5C3415E642AE73F1C873A5A/?imw=5000&imh=5000&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

I haven't had these brick on me in another layout yet. The thin hallways/bridges here seem to glitch them out.