You can't place water in double-slabs, they just vanish bcs slabs are water-source, but double slabs destroys the water bcs is a full block, so you can't on it, or around.
MC-131021 Placing water or lava on top of a top slab, the slab is waterlogged / the lava is not placed
MC-126906 Buckets cannot be emptied against blocks with the state waterlogged
MC-144514 Water buckets won't interact with double slabs
Bug
