Steps to reproduce
Set diffculty to hard and weather to rain (so zombies don't burn).
Spawn a zombie.
Drop a shulker box on the zombie.
Drop a fish on the zombie.
If the zombie does not pick up either one, kill it and go back to step (2)
Expected result
A zombie picks up the shulker box.
Observed result
A zombie that will not pick up a shulker box may nevertheless pick up a fish.
Mobs such as: Zombies, Drowned, Foxes, and other mobs that can pick up items, can no longer pick up shulker boxes.
Can confirm. Additionally, this has not been fixed as of the latest preview (1.20.60.21)