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:falling_block,Time:1b}]}/summon Boat ~ ~ ~ {Passengers:[{id:tnt,Fuse:1000}]}/summon Boat ~ ~ ~ {Passengers:[{id:tnt,Fuse:1000}]}Suggested Fix
Make non-living entities always face the front of the boat.
This is NOT a duplicate of MC-90148. In MC-90148, the issue is that the heads of entities in spinning boats continue spinning AFTER you get out. This issue is that WHILE you are riding the boat, the minecart glitches and spazzes out. It is different!!
Edit: Nevermind, I updated the report and added more details on the issue, which is kind of different from MC-90148.