This ticket should probably be split up, so that we have one ticket for lava not being able to be placed, and one ticket for waterlogging while looking at the top face of a top slab / bottom face of a bottom slab.
This is probably intended, that just is how waterlogging works. If you right click a waterloggable block with a water bucket, the block will be waterlogged and if you place it by aiming at the corresponding side, it will also be waterlogged. I don’t think, this is a bug.
Confirming for 1.13-pre1.
I suspect the water aspect of this might be intentional? For lava though, and for double slabs. it makes no sense.
Also, this seems to be related to MC-126906.