It Works as Intended,you are pushed out of the Boat if youe boat is over Soul Sand, and the Video you posted Shows a Bug that was Fixed, in which Boats Traveled Upstream.
Can confirm for 18w11a
Only Happens When a mob is teleported just the moment it is spawned.It happens always regardless of where you teleport it and affects all entities, not just Turtles or Mooshrooms.
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EDIT: In the First Screenshot I spawned the mob using a renamed Mob Egg
Actually the Forward Slash " / " Key is used to open the Chat UI Faster to type commands not the Hash key "#"
can confirm for 18w10b, but as it appears The child Mobs are not affected by this bug and neither the profession of the Brown Coat.
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Yes, they fixed it in 18w10a with the new Texture and no spawning in End. Said that 'It made phantoms less derpy'
I also request thee to make Spawn Chunks entity processing back again, Please.