I'm guessing it's something to do with his 7* Release.
He has a node that applies Power Lock and Heal Block whenever a poison fails to apply due to chance.

Both Poison immune champs as well as those not immune are permanently affected by a power and heal (b)lock.
This was not a problem during the Original run or the amped up version 11 months ago.
So any champ that relies on specials is gonna be gimped. Someone like Mantis who can completely shut down his debuffs is and option I think.