When a baby piglin (or other small mob) steps into the block that is holding a fence corner, it seems unable to move.
Notice how the cursor is pointed onto the block that both the piglin (picture with piglin) and the fence are on, the piglin needs to be on that block for the bug to happen. The bug seems to be caused when the mob's whole hitbox is in the same space as the fence corner (picture with zombie)
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Can confirm. Seems to happen on the sides of fences as well.

Can anyone check if this affects all variants of baby zombies, and other baby mobs? I'm pretty sure that baby zombies are affected by this.

Same issue that causes chickens to get stuck in fence corners?

@DrownedZombie can confirm that other mobs can get stuck in fence corners, all mobs that get their whole hitbox in the space that the fence corner is in as shown in the picture with the baby zombie. Should I update the name to "Small mobs get stuck in fence corners?"

No need, I've already updated your ticket.

Seems to be fixed to me, I can clearly punch baby husks without breaking the fence in creative.

@DrownedZombie The bug still seems to exist. I just tested it again on 20w09a. The best way to test it is with a baby villager and a NoAI zombie by it.

In 20w12a

While this bug might be fixed for fence corners I still experience baby villagers getting stuck in straight sections of fence and walls if they are pushed into them.
They will get free if pushed by the player or another mob or if the fence/wall is broken or when they grow up to adult villagers.
This time the baby villager got stuck after I punched it into the fence and it stays there indefinetly. Won't move when it's nighttime despite there is a bed only two blocks away, nor will it move towards dropped food or run from hostile mobs or if a bell is rung.
See screenshot
I could not find any current bugreport pertaining to this. I will create a new report if necessary.