In the Urban Battleground Fractal the enemies by the Mistlock Singularity are Invulnerable.
You can try melee or range, and they still take no damage.
Whether you are on the bridge, or in the trenches they take no damage.
Even AoE’s seem to not damage these enemies.
They used to take damage so this is a glitch due to changes in script behaviors.
I suggest checking how these mobs were instantiated.
Specifically if their collision was enabled.