The bug

When a mob had a passenger, he don't move, he don't attack players (for hostile mobs) but players can interact with them.

How to reproduce

• Spawn a shulker riding a ravager

/summon minecraft:ravager ~ ~ ~ {Passengers:[{id:shulker}]} 

→  The ravager don't attack you and don't move

Final note

When you kill the shulker the ravager is no longer inactive and attack you. This happen with all mobs that have a passenger and the other mobs can attack the mob that he innactive.