When placing water on top of a slab that's on the upper half of a block the water will be placed underneath

Slabs can be placed in either the upper or lower section of a block. There's a relatively new feature that allows half blocks of water, but when you place water on top of a slab that is set to the top half of a block the water will be placed beneath the slab as a half block of water instead of being placed on top of the slab.