i noticed that ironwood stairs have this issue as well. havent gotten around to checking other blocks in the mod.

I can't seem to reproduce this. Do you have any more information you could provide?

This might be caused by Optifine. Do you have it installed? #142

This does in fact seem to be caused by optifine. I'll see if this happens with other mods' stairs and stuff, and see if anyone else has found a solution for this.

I managed to find a fix for this. I plan on releasing an update with this fix and more later today.

The update has been released. It won't fix already placed blocks, though. You'll have to replace them and make sure the chunk's lighting updates. It might be a bit wonky at first, but once the dark spots are gone they should never re-appear.