Slabs at buildheight (nether)

When trying to place top slabs on bottom slabs at max buildheight, the top slab wont be placed. When i try to place a slab it gets "consumed", when i place a slab as a bottom slab i get the "consumed" blocks back. When getting another stack from my inventory manually, all the "consumed" blocks return, however, they do not come back if i use the MWHEEL hotkey to get the slabs out from my inventory