I killed everyone except the whispering shade (immune shacker boss). They revived, but the right two enemies sprites didn’t show up. I killed them all again and the top sprites didn’t respawn either.
The missing sprite enemies do not respond to attacks(invisible), they do die with spells.
After killing the last one with a normal attack the others respawned with their sprites intact. However, one of them turned into a whispering shade like the bottom center boss.
It is repeatable upon reentering the realm.