I would include a link to a video but I cannot record bot matches. I was testing Swain (Northern Front) and noticed that upon finishing ultimate a LARGE black textureless coat clips and hovers over Swain's model for a short duration. I think it could've been an issue with stepping in a bush while ulting, but I cannot tell for sure what causes it.
My team was Swain (Me), Kled, Nasus, Azir, Tryndamere fighting a bot team consisting of Kayle, Shyvana, Rammus, Alistar, and Cassiopeia.
The steps were:
1. Pick Swain (I used Northern Front Skin though it may affect all skins, i'll check soon if it is only this skin bugging out)
2. Get resources for ult (I had 5 fragments each time I ulted)
3. Use ult, casting demonflare whenever prefered.
4. The bug should appear directly after ending the ult; if not, walk into a bush while ulting first to see if that's the cause.
Speaking of which, Shyvana's AI seems incredibly busted to where she just wanders top-lane regardless of the current state of the game. Even when her whole team is dead and inhibitors are down she will rush top lane regardless. Additionally, she doesn't seem to buy and items outside of Doran's Shield.