When a command block tries to teleport a boat rided by a player, the output says that it teleported successfully, but the boat stays at the same place.

Here are the steps to reproduce this bug :

1. Place a repeating command block, always active with the following command : 

   teleport @e[type=boat,limit=1,sort=nearest] ~ ~1 ~
   

1. Enter the boat
2. Try to move the boat around