Lighting doesn't work for Custom Blocks

When creating blocks that are smaller than a full block that emit light, like my coloured lantern, the block isn't illuminated correctly.

This becomes very obvious when using the same textures as the regular lantern or when trying to make different coloured fire

Update: You can turn ambient occlusion off in material instances which fixes the dimming of custom geometry, though this isn't listed in the documentation

Expected Behavior:

 - Blocks with geometry smaller than a full block illuminate correctly

Actual Behavior:

• Block is 'dimmer' than it should be

Steps to reproduce:

1. Create a block with geometry smaller than a regular block
2. Change the blocks light emission value to 1.0
3. Observe the block is dimmer than it should be