Not a bug, if you read the description of U-turn on mouseover, it says "The user does not switch out if there are no unfainted party members, or if the target switched out using an Eject Button."I don't know if it's a bug or not but I wasn't able to switch after using U-Turn with my Landorus-Therian on a Tyranitar with a Eject Button as item, we were playing Turn 1 so I had all my 6 mons.