Their Hitbox is correct, but their textures are not matching with their hitbox.
I tested all slabs from this mod and those are the only ones not properly working.

Here - few screenshots to show it:
All Slabs from this mod (Top Layer Third from Left and Bottom Layer Fourth from Right)

I found this mistake because I was placing all the sand related blocks from my modpack to see their textures... If I was doing it to any other block type I would probably not have found this bug.

I was not testing fully this mod so I don't know of other potential bugs.