The bug

Light-emitting blocks in the world will always appear to be that specific light level, even if they aren't transmitting light (e.g. due to being occluded by tinted glass). However, they do appear dark:

• When displayed in a minecart
• When held by an enderman
• In item form (held by the player in first person, held by the player in third person or another entity, in an item frame, dropped on the ground, etc.)

How to fix

This can be fixed by using the new light_emission parameter for model files - simply set it to the emitted light level in question for that block and it should appear correctly under these conditions. A resource pack to fix this issue is attached.