Unshaded model appears shaded in the inventory

Unshaded blocks models appear shaded in the inventory GUI / hotbar / hand

What I expected to happen was...:
The model should look unshaded like it does in the world when placed

What actually happened was...:
The model looks shaded

Steps to Reproduce:

1. Get a resource pack that adds an unshaded block/item model to the inventory
2. Observe the shaded model