Breaking heads (skulls) in survival that are on the side of a block will not drop the head item. This includes Dragon Heads (they generate naturally on End Ships, on the side of a block). The tool used (hand or pickaxe) does not matter.

Heads placed on top of a block will break properly and give the head item. Edit: heads placed on the bottom of blocks give the head item too.

I have tested in 1.13 vanilla and without plugins. In vanilla the heads do drop the item.

Steps to reproduce:

1. Make a new world
2. /give @p minecraft:wither_skeleton_skull 10
3. Place a skull on the side of a block
4. /gamemode survival @p
5. Break a skull that was previously placed on the side of another block - it will not give you the item

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