I couldn't find any reports on this already but skeletons/pillagers (and likely any other ranged mob) when a passenger of a boat, cannot hit a player in the front seat of the same boat. Rather than hitting the player in the back, they instead shoot down into the ground?

Perhaps this has something to do with the similar bug where sometimes skeletons will shoot straight down instead of hitting the player when in close quarters combat and their hitboxes are colliding?