Teleporting players with relative coordinates behaves like before 16w21b when used in a command block [minecart]

According to the changelog, /tp with relative coordinates now teleports relative to the executor, not the target. However, when command blocks teleport players, the command seems to teleport relative to the target.

to reproduce:

1. create a superflat "redstone ready" world
2. Place down a repeating command block
3. set the command to

/tp @p ~ 58 ~

4. set the command block to "always active"
5. you will notice that, despite reporting the correct coordinates in the logs, the player will be teleported according to their current coordinates.