This is happening frequently for me so I’m reporting it. In some echoes, in the middle of combat the monsters in the instance suddenly become immortal.
The mods on the echoes varied from “slow” to “30% endurance” to “deal increased damage” etc but nothing that would make them too tanky for my build, and in some cases I cleared 90% of the echo before the mobs suddenly became immune.
Whenever I leave the instance and come back to the same instance new the mobs are immortal from the beginning, however if I restart my game and go back to the same exact echo I can kill them again.
Cheers