Kayn Tower agro bug

So I came across this bug when I was playing against a Kayn. Kayn ulted me under tower and dropped tower agro instantly as intended since its him being untargetable. But when he comes out of his ultimate, he deals damage, and so the tower should theoretically target him again, but it does not. This resulted in him leaving the dive pretty much freely and me losing the trade even though I set up multiple Shaco boxes to fear him (sorry to those who hate Shaco top). This didn't just happen once, but multiple times where the tower didn't want to agro him if he damaged me with his ultimate under tower.

I think this is because he is still untargetable for a period of time even after his ultimate ends, but I think its a bit too long because it makes his dives much more forgiving than what it should be.