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.
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.
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.