Dispensers placing pumpkin to create snow golems or iron golems or placing wither skeleton skull to create wither cannot replace grass, snow, fire etc.

The bug

Dispensers are able to place pumpkins when it spawns a snow golem or iron golem then and they can place wither skeleton skulls to spawn a wither. However if there is a block like grass, snow or fire etc. in front of the dispenser, it won’t be able to place it even though the player could. It can only place these blocks if the block in front of them is air. This works fine with placing shulker boxes.

How to reproduce:

1. Place to snow blocks on top of each other
2. Place a snow layer on top
3. Place a dispenser facing into it
4. Put a pumpkin inside
5. Power it
6. The pumpkin was not placed, because there is a snow.
7. Break the snow
8. Power the dispenser again
   The snow golem is spawned.