As soon as levelled to 89 in Arena, I did no damage to any mobs. I had to quit the Arena.
I then tried to do Monoliths, and once again every single mob is immune to damage.
I then went to various story modes. I could do damage to all story mode mobs.
It is probably related to level 89 mobs. I could not test higher level mobs.
Edit: I also tried to do a new Arena and once again I did no damage to any mobs. In addition, they do damage to me, so it’s not like they don’t exist. I tried about 10 Monolith runs with the same result.