The bug

Dispensers can place pumpkins to spawn iron golems and snow golems and they can place wither skeleton skulls to spawn withers. However this works even if there is an entity standing inside. For shulker boxes this works fine. With the pumpkin the mob would immediately be spawned (without using MC-137248), but for the skull you would be standing inside it. Relates to/was discovered while testing MC-217270.

How to reproduce:

1. Place soul sand to spawn a wither
2. Place a dispenser facing into a block on top of the soul sand
3. Put a wither skeleton skull inside
4. Stand in front of the dispenser so that you would be inside the skull when placed
5. Power the dispenser
   The skull was placed inside you.