The Envoy Part Ii Key Not Spawning Problem

So ive gone through the avos temple like three times now. Each time when after i throw the drop from the last wave into the hopper and get dropped into the water in the final room no key spawns and i end up stuck in the room. this is really frustrating please help

 

I believe that there is supposed to be a boss that drops the key in that room, although I may be wrong.