Possible swain bug

I recently was facing a teemo as swain. I got the teemo really low but was out of mana and he went into the bush and stealthed. I figured using my ult in the bush would still hit him however it did absolutely nothing. I thought maybe he left the bush and I somehow didn’t notice but after a few seconds after my ult ended he zoomed out of the bush to his tower. I’m not sure if this was a bug or not but it didn’t seem like that’s how swains ult should have interacted with teemo that way. Just wanted to give anyone a heads up if this is not how it’s intended to work. Also I want to add that it most definitely should have killed him.