This may seem like an oddly specific bug, however it's become frustrating to me due to the nature of it. The newest update, Java Edition 1.21 introduced a feature where entities such as horses, minecarts and boats can be ridden through portals, maintaining the player in a riding state on top. This feature also finally allows entities to be warped using /tp with players riding them, which is very useful to a map-maker like me. However, one thing that I've noticed over time is that there is an inconsistency present. Attempting to warp an entity like a boat only works roughly 50% of the time, while it works 100% of the time when there is no player in it. I can not seem to find any external reason why this occurs.

This bug can be recreated simply by using the command /tp @e[type=boat] ~ ~0.5 ~2 while in a boat on water, and seeing how often the boat actually moves while a player is in it, vs. when they are not. The picture attached is a screenshot where I spammed the command above in a boat, and you can see on the far "Z" coordinate on the command how often the boat with my player in it actually successfully was teleported.