April 23rd, still happening…
If Blizz doesn’t want to take the time to make sure these work properly then please stop making dumb gimmick fights.
April 23rd still happening 
Just happened to me, April 25th.
Why is this dungeon still being given on keys with a bug that kills runs over a month after discovery?
April 24th, dungeon is still bugged. Why has this not been fixed yet… add a npc to talk to in the room to reset or something. Another waste of my time on top of all the other time wasting mechanics in this game.
Still busted, April 24th. Had a key completely borked because an instrument never spawned.
4/30/22 @ Mythic 15 - guitar didn’t spawn. This is still happening.
Same problem, 4 instruments only, could not start encounter, killed the key…
Same issue today. No guitar.
same problem, lost my 15+ key, no guitar
Thanks for this, the same workaround worked for me inadvertently just now. The encounter bugged, and we couldn’t reset it. Someone had to leave, and then it suddenly started. Since it was a +2 we just finished up the dungeon, and the guy who had to leave got one of the items