Summoning a Boat with x and y or ~ causes “Sinking” Appearance

Summoning a boat while specifying an angle causes the boat to be seen at unusual steepnesses.

This can be seen by either specifying an x y or by just doing ~ ~.

The correct behaviour can be seen with the Minecart as specifying an x y or doing ~ ~ causes just rotation, not steepness, to change.

Example Command:
/summon boat ~ ~ ~ 100 100