The bug

In 1.12.2 and earlier, all head blocks were grouped together under a single ID: "minecraft:skull", and differentiated via states. 1.13, due to reforming block IDs and states, split each head into its own unique ID. However, all of the head blocks still reference a model file using the outdated, pre-1.13 name for all heads: "skull.json".

How to reproduce

1. Extract the minecraft.jar file by your preferred method
2. Navigate to the assets/minecraft/blockstates directory
3. Open any json file relating to a head or skull
4. Note that it points to block/skull, rather than something more accurately reflecting the block's name
5. Now navigate to the assets/minecraft/models/block directory
6. Note that there are no dedicated models for each head, only skull.json

Expected result

As with most other blocks, heads would have a unique model file for each type.

Actual result

There are no head models. There were never any head models. Only skull.json.

How to fix

The resource pack attached to this issue fixes this issue entirely by creating six models, one for each head type. (Both the wall and floor variant blockstates file reference the same model file for each head type.) In addition, the resource pack performs a partial fix for MC-1874, by adding new texture files for each head to use for particles (cropped from the respective entity texture files, with some additional modifications for the ender dragon). Programmer Art versions of these textures are also present in the zip.