When trying to place water on top of a water locked block where the top face of the block is complete (slab on top or upside down stair), the water is placed on the water locked block and not on top of it, this also happens if water is placed on the back of the stair; in addition and the most "logical" when also removing water from a water locked block is by the section where there is water and not removing it in full or partial faces.

I suggest that if this is the expected behavior, implement the shift to place water on a full or partial face of a water locked block, although the section where there is water would be the only place to put water.