The bug
When you place a bucket of water on a top slab, the water is placed inside the slab instead of above.
Example
Here is an example in video: https://www.youtube.com/watch?v=5gcZIO6C74k
Additional information
This bug causes that you cannot place lava over a top slab.
Linked issues
is duplicated by
relates to
Attachments
Comments

Affects 1.13-pre2

Affects 1.13-pre3

Can confirm for 1.13-pre4.

Double slab bit fixed in 1.13-pre5, top slab issue remains.

As of 1.13-pre6, only the top line applies.

Affects 18w30b

Still affecting 1.13.1's full release. Also, why is the middle statement not crossed out?

18w47b, only the top line remains valid

Top line affects 18w49a, everything else remains fixed
Video recording of water and top-half slab bug: https://www.youtube.com/watch?v=5gcZIO6C74k
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.
Lava can be placed without any issue in 1.16.2
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.