The bug
When you empty a water bucket while looking at the solid face of a partial block like a staircase or a slab, the water becomes waterlogged in the partial block instead of being placed above it.
There are 2 screenshots below of a 3×3 platform of slabs, one before emptying the bucket and one after emptying it. You would expect the water to be placed on top of the slab if emptied on the top face and waterlogged if emptied on the bottom face. Instead, though, the water is waterlogged, which is not the intended behavior (I think).
Linked issues
duplicates 1
Attachments
Comments

Affects 1.13.1.
Mention that it also affects blocks such as fences, on parts that protrude out of source water.
The ticket shown that this duplicates is not the same as this one, ie: does not duplicate. That has to do with water flowing through slabs/stairs, this ticket addresses the fact that you cannot place water adjacent to such blocks without it becoming waterlogged.
Affects version 1.13.2-pre1. Impossible to place water adjacent to water log-able blocks with the use of helper blocks.
Still affects version 1.13.2.

Duplicate of MC-131021.
I think this is a duplicate.
I'll comment here anyway since I see no reason why we cannot place water against a partial block with being forced to have another block in place to do so. Just like ow pumpkins used to be. Shift clicking would be one option.