The Bug:

Minecarts that have passengers within them riding mobs can move automatically when not on rails.

This seems to happen for mobs in which their AI is overridden by default, for example, chickens and spiders.

Steps to Reproduce:

1. Summon a minecart that has a passenger within them riding a mob, for example, a chicken jockey by using the command provided below.

   /summon minecraft:minecart ~2 ~ ~2 {Passengers:[{id:"minecraft:chicken",Passengers:[{id:"minecraft:zombie",IsBaby:1b,ArmorItems:[{},{},{},{id:"minecraft:iron_helmet",Count:1b}]}]}]}

2. Observe the minecart's movement for a short while.
3. Take note as to whether or not minecarts that have passengers within them riding mobs can move automatically when not on rails.

Observed Behavior:

Minecarts can move automatically when not on rails.

Expected Behavior:

Minecarts would not be able to move automatically when not on rails.