Vehicles controlled by a player don't get launched by slime block pistons consistently.

When using redstone to activate a piston pushing a slime block up, any mounts on top of the slime block do not always get launched. From the players screen it looks like they hit their head on something invisible. This behaviour is inconsistant, seemingly at random, and specifically effect players controlling another entity. Players by themselves work as expected, and ridable entities that are NOT being controlled by a player work as expected. Taking a saddle off a horse and riding it does not produce this bug, so it is almost certainly related to the players control, as also shown by the fact that hostile mobs (e.g. zombie) controlling a horse never produces this bug.