Basically according to the devs there, in vanilla minecraft the bottom-slab is considered a transparent block which is a bug that they fixed. I don't know if you have ever noticed the lighting bugs under a slab-ceiling? They "fixed" this issue but gave a special exception to chests so they would still open with an "upper" slab since it makes sense given there is a space, but not a lower slab.
I hope this settles the controversy here. The dev there says it has been this way in forge since almost the beginning and another of my friends that actually sets up our forge server confirmed this is indeed the case. It is possible that in the case of large servers or modpacks they reverted this part of base forge since it confuses players used to vanilla? That could explain your experience.
If it was a bug in the game, they would have fixed it long long ago, it has been there since slabs, and I am sure mojang kept it for a reason.