It already happened 3 times to me now in different runs. In the Hexahedron, after the 3rd or 4th time rotating the cube at the end when you have to step in the middle to be lifted up...what happens is that at the peak in the air it flashes without changing the room. The guardian(s) fall down again in the middle plate, Toland stays there as if nothing happened but now you're no longer being lifted up. The encounter in the room seems finished, there're no more goblins spawning and therefore no more knights spawning. Basically you can't leave the room and you're stuck there forever. You have no other option but wiping and restart the whole Hexahedron encounter. However even in that case it might happen again.