The Bug
Entities can't push shulkers riding in minecarts.
Steps to Reproduce
Summon a shulker that's riding in a minecart.
/summon minecraft:minecart ~3 ~ ~ {Passengers:[{id:"minecraft:shulker"}]}
Attempt to push the minecart.
→ ❌ Notice how entities can't push shulkers riding in minecarts.
Expected Behavior
The expected behavior would be that entities can push shulkers riding in minecarts.
Linked issues
is duplicated by 2
relates to 2
Attachments
Comments 20

But other minecart can push it.
But it is a related.
Can confirm in 20w48a. Feel free to use this command if you wish to reproduce this bug yourself.
/summon minecart ~ ~ ~ {Passengers:[{id:shulker}]}
Can confirm in 20w51a.
Affects 1.17
It still affects 1.17.1. The shulker can be moved if there is another minecart behind it. Cannot move shulker while in boat as well.
I am able to confirm this behavior in 21w38a. Here are some extra details regarding this problem.
The Bug:
Entities can't push shulkers riding in minecarts.
Steps to Reproduce:
Summon a shulker that's riding in a minecart.
/summon minecraft:minecart ~3 ~ ~ {Passengers:[{id:"minecraft:shulker"}]}
Attempt to push the minecart.
→ ❌ Notice how entities can't push shulkers riding in minecarts.
Expected Behavior:
The expected behavior would be that entities can push shulkers riding in minecarts.
Can confirm in 1.18.1.
Can confirm in 1.18.2 and 22w15a.
Probably caused by the hitbox of the Shulker which is bigger than the one of the Minecart, see MC-13937