The marble slab blocks light when it is not a double slab, causing weird lighting issues, which can be seen on the surface of a lower slab and the block faces adjacent to the upper slab in the screenshot below.

Mod version: 1.12.2-1.8.5

I don't mean to be rude, but this is far from a complex issue
You need to remove your call to Block#setLightOpacity at BlockMarbleSlab.java#L57
Lighting opacity and AO is already handled by BlockSlab#isOpaqueCube

Should have dropped that into the Discord when discussing it several days ago, I just tag things as discussed.