When attempting to place water on top of or next to non-full blocks the water does not place as presumed it should be intended.
When trying to place water next to or on top of a double slab the water will refuse to place at all.
When trying to place water on top of an upside down slab or upside down stair the water will place inside the block instead of on top.
I've recorded a quick video demonstration here: https://youtu.be/-m4VpiSgwe0
Confirmed, can't place water on top of waterloggable blocks.