THIS ISSUE ALSO OCCURS ON VANILLA!
http://prntscr.com/lzv9bg
You can't build a half slab above another half slab at 255 height by clicking on top of the other slab.
It is possible to build a half slab that is connected to another block though: http://prntscr.com/lzv9jw
It is also possible to build that other slab by clicking on another block http://prntscr.com/lzvb4h. If I click on the dirtblock with a slab the slab gets placed. http://prntscr.com/lzvbda