This issue clones:
While testing MC-99307, I discovered a rare bug that doesn't always happen where boats on mobs are extremely glitched out. There was an enderman, and put a boat on the enderman and the boat started glitching out very bad, I started sinking into the ground and my player was glitching back and forth. The enderman was inside a sand block for some reason.
MC-167299 for an example of this bug.
Due to MC-192836, this issue is very difficult (but still possible) to reproduce.