Most bugs encountered in League of Legends are quite tilting, some of them like the bug splat are even infuriating. However, some weird visual glitches sometimes happen without affecting too much the gameplay. Here is a good and quite hilarious example of what you can sometimes expect into the Rift.
Somehow — perhaps because they're both Void champions — it appears that Kai'Sa morphed into an awkward version of Rek'Sai and started to burrow and lurk under the ground. While some of these visual glitches aren't that rare, it's important to underline that The_Winkles, the Redditor original poster, confirmed that everyone in the game could see the bugging sprite.
This would suggest that this bug doesn't come from The_Winkles but from the game itself.