Mobs stop moving when an entity is riding them

I made my custom entity ride another mob, and the ridden mob stops moving. If the ridden mob is a zombie or has zombie AI, it will still attack the player but won't wander around. If the ridden mob has custom AI, I can't get it to move at all. Putting Spider AI on the ridden mob works (I think because of spider jockeys), but I don't want the mob to climb walls. how do I get this mob to wander normally with a mob riding it?

I'm in 1.16.5 btw