Mob, on which sits any other entity, even the armor stand, it can not be pushed, it can not be moved, such mobs can walk through each other. Exactly the same mob without a passenger, it works fine.
demonstration on video

/summon vindication_illager ~ ~1 ~ {Team:"BLUE",Johnny:1b,Passengers:[{id:"minecraft:pig",Team:"BLUE",NoAI:1b}]}

/summon vindication_illager ~ ~1 ~ {Johnny:1b,Silent:1,Team:"BLUE"}