If you look at a top-level half slab from the top looking down at it, you would expect the water to be placed on top, one level above the half slab. Instead, it appears to be placed in the block waterlogged, so it flows beneath the block.
MC-144434
Placing water on a top-level half slab creates source block inside block instead of on top
Resolved
Duplicate
0
TeamCobra
(Unassigned)
Unconfirmed
Minecraft 1.13.2
Retrieved
i dont work at Mojang or anything but i dont think this is a bug. when you place a water block you would expect it to fill a block in your line of sight that can be filled with water and its highest priority is the farthest block in your range and then the block infront of that. you dont expect it to fill the block infront of your face instead of the block on the ground that you are looking at. while this may be annoying in some circumstances i dont think its a bug and fixing it would disable you from manually waterlogging all half slabs on the top half