Slabs don't behave correctly

Isue:

When i atempt to place a top slab, i get a botom slab. And when i atempt to make a full block, it just places a top slab besids it. What is going on with my game? Or is it me who is doing wrong?

 

What is suposed to hapen:

I should be able to place top slabs.

 

(Had to trim the video to fit the 10 MB limit. If i can eddit then i will send more to display the problem)