Half slabs won't be half slabs

Even though I place each slab down the same way every time, it's more likely to be placed has block rather than a half slab. When I place it on flat ground, it's likely to be placed as a block. When I place it on the side of a standing wall or a free-hanging half slab, it's more likely placed as a half slab, but sometimes still places as a block. Not sure if this is a new feature or bug, but it doesn't seem right to me. I have to break it every other time to re-place it.