If a class joins and fills a slot and then leaves/gets kicked out, the slot will be empty again but the class required will be locked to that class (bug/oversight?) even if that slot was previously all classes. So it's possible that a blm tried to join and got kicked out lol.

Unless you're talking about t4 then I guess blm would be great there but there's only 2 aoe heavy phase out 6 and the aoe part isnt even that important, not even before echo. Monks can fall behind on these 2 aoe phases and shoot straight back up if he knows what he's doing and never drop a stack.