Zombies only attack blocking carts on hard (floors and paved) surfaces?

needs confirmation, but it sure seemed to me like zombies would only attack my wooden box, wooden frame cart’s box when it was on hard surfaces, but if I drag them AND it over dirt/grass they would happilly climb on without ever doing damage to the cart in the process/instead.

Seems like any time they are on hard surfaces they like to get a hit or two on the cart when its being used as a mobile shield.

I can’t think of any reason for this to be so unless its a bug.