UPDATE: Klein has returned to the original post to add yet another edit, announcing that the problem is more complicated than we originally thought. Interestingly, Klein name drops Xayah, Kled and Fizz as other champions with this particular issue. A hotfix is currently being worked on.
EDIT 2: Quick update on this! Turns out we can't actually fix this on individual champions (so Xayah most likely has this bug still). We confirmed it on Kled and Fizz as well. We're currently working on a code-change to fix this internally for turrets and are hoping to hotfix that out as soon as possible.
Kayn, League of Legends’ newest jungler, hit the Rift Wednesday. As always, the release of a new champion is very exciting. We here at the Rift Herald even have two guides for the new fella: one for how to play him and one for his passive. But that new champion smell won’t last long, as Kayn has a pretty nasty bug when diving towers.
Reddit user biggooser669 posted a video to the League of Legends’ subreddit of Kayn diving a Garen with his ultimate, becoming untargetable (which is supposed to happen), jumping out and killing Garen under tower without getting aggro (which is not supposed to happen).
Kayn clearly attacks Garen while under tower, and yet the tower ignores him, blissfully happy to keep shooting at blue side minions. Kayn walks away unscathed and then Garen probably rage quit the game.
Gameplay designer of Kayn and friend of the site, Daniel Z. Klein, commented on the thread.
He is untargetable when he deals damage so that part I understand. Why he doesn't take aggro afterwards though I have no idea! That indeed looks like a bug. We'll try to repro and fix tomorrow. Thanks for the report.
A few hours later, Daniel returned with an edit to his original post, which gave a little more clarity on the issue in question and a potential fix.
EDIT: I believe we fixed this. We'll explore the possibility of a hotfix. From early indications, the timing has to be incredibly precise (I won't go into exactly how to make it happen here for obvious reasons), so while this is officially Broken As Fuck when it does happen, it doesn't currently look like you can make it happen on purpose. Will update when the fix is in!
As of this writing, the hotfix has not gone out for Kayn. However, it sounds like a solution is in the works and should be here soon. This could potentially result in a temporary disabling of the new champion, although the issue sounds fairly rare. We will keep this post updated based on Kayn’s availability and the incoming hotfix.