It’s pretty straight forward and very easy to reproduce. When you duel someone and use the skill Telekinesis on them and go on to hit them on the ground with it, damage will pop up but not register onto the person. The damage that pops up is also very inconsistent. It will do 10 dmg on one hit, and over a thousand on another.
The cause may be the fix implemented to keep Telekinesis from hurting people outside the duel. The fix may of unintentionally caused it to also not do damage to the players inside the duel.