Falling block entities and tnt entities do not rotate with a boat when it rotates. Armor stands and minecarts, on the other hand, behave like a living entity, and living entities are currently broken in boats, they glitch and spaz and rotate all over the place.

Commands to reproduce:

/summon Boat ~ ~ ~ {Passengers:[{id:falling_block,Time:1b}]}

/summon Boat ~ ~ ~ {Passengers:[{id:tnt,Fuse:1000}]}

Suggested Fix

Make non-living entities always face the front of the boat.