Describe the bug

1. What did you expect to happen?

I put my head into the underside of a stair block.
I expected to stay the same light level.

2. What actually happened instead (i.e. what was the bug)

My entire body turned dark.

3. If it was a crash, upload the full log file (latest.log)

No crash.

4. If they would help to explain the issue, include screenshots

Player model in vanilla stairs versus TFC stairs.

Item entities turn black while inside stairs.

To Reproduce

1. Place some slabs on the ground
2. Place some upside down stairs 2 blocks above the slabs.
3. Walk on the slabs so that your head is in the stairs.
4. Your body turns black.

Note: any entities that fit into the air space of TFC stair blocks turn black, such as items.

Meta Info

• TFC Version: 0.21.4.DEV
• Were any other mods included? Can you reproduce it without these other mods?
  No other mods.

I'd like to point out that the method in which vanilla decides to assign useNeighborBrightness is to check, after registering vanilla blocks, if block instanceof BlockStairs || block instanceof BlockSlab, which conveniently only applies to vanilla stairs, and is also in the last place I'd think to look.
¯\_(ツ)_/¯