Cho'gath true damage bug on Zac

I was playing a custom game with my friend, he was on Cho'gath and I was on Zac. When the indicator appeared for his feast to kill me and he used it I would be put into my passive as expected. However when he attacked my blobs using basic abilities it would deal over 2000 true damage to the blobs and on the death recap it would state that I had either taken the true damage from his ult or from his passive ( which doesn't deal damage). I'm pretty sure Cho'gath shouldn't be able to deal 3k true damage with his q or w even if it is to Zac's blobs. Images of death recap.