Some entities with riders not getting teleported on every execution of the /teleport command

1.6.0.0_x64_UAP.Release_Test
Release_980905

Summary:
Using /Teleport on boats (or saddled horses, (and other saddled animals?)) with a rider only works "every other" time. Ie, the first time it is used it will work, the second time it will not, the third time it will.

Oddly, minecarts and non-saddled horses work every time.

Impact:
Requires us to execute a teleport on the entity/player combo a few times to make sure that it gets teleported.

What is the impact to players/partners if we don't fix?
Redundancy in commands that may also appear to be buggy when the bug gets fixed

Repro Steps:
Creative mode,
grab a boat,
put boat on ground (not water, /teleport doesn't work on boats with players riding them while in the water....making another bug for that..),
get in boat,
/teleport @e[type=boat,r=2] ~ ~5 ~
/teleport @e[type=boat,r=2] ~ ~5 ~ a second time. Repeat to observe the pattern of the command working and then not working.

Observed Results:
/teleport @e[type=boat,r=2] ~ ~5 ~ should teleport the boat/player up 5 in the y axis, but only every other time.

Expected Results:
It is expected that the command would work reliably every time.