grave charm
Played the siege of Mirifuir to the end and then it kind of just broke. I managed to bully the third mage guy into a corner and somehow his two buddies didn't spawn in and now he's dead and the castle is perpetually shaking and I can't complete the siege because the brazier isn't lit up and I can't use the torch.
Also, the two initial casters both dropped their keys twice over - once when falling to a knee and a second time when fully teleporting away.
Leaving the bossfight area did not cause the missing mages to respawn. Respawning also did not cause the missing mages to respawn, but caused all the regular enemies to respawn, which it obviously really shouldn't be doing during an ongoing siege.
To be clear: The castle only shakes in the boss arena itself, it's not the entire area.
Gonna have to quite because this is well and truly glitched. Sucks to have the siege break on you in the literal final stage of the fight.
In case it matters: I stunned the Petra guy with Bromeus' Wevetroot ability right at the start of the fight, which may have broken his summoning of the other two blokes.
cedar steeple
+1; Other circumstances, same issue
warped fable
For me, the third one stood at the back of the arena in the dark (had glowing eyes! was kind of cool) until I attacked them and the other 2 didn't appear. I assumed this was meant to happen but the braziers didn't light up after the fight was over.
dawn nimbus
+1 the key fell into a gab in the stairs
cedar steeple
+1 the key fell into a gab in the stairs
Also +1 on this
glass cobalt
+1 to this. I got to the final encounter with the three bosses. Killed the staff one. The fire one teleported away and never came back.