I have recently created a new block to my mod which is based on a slab.
The problem appeared when I stacked the slab and it became a full block. Despite having 0 and 255 opacity (I tried both) the full block remained transparent, letting light through. There is a way to use procedures to manually replace the block if a slab is placed on top of another slab, but the feature of creating custom blocks was created to ease the process of creating slabs, stairs, etc. It would help me a lot if you could take a moment to look at it.
This also appears to happen with stairs to, as far as I've checked...
I created a sample workspace and shared a clip for you to check the bug: