The bug
When the block data values were removed, and fully replaced with block states, the ability to set the block state on superflat worlds was lost. You used to be able to set the data value in the preset.
This preset is expected to work and place top slabs, but it doesn't.
minecraft:stone_slab[type=top];minecraft:the_void;decoration
No format actually exists anymore to set block states in the superflat preset.
This goes for both the world generation menu, and custom dimensions.
Linked issues
blocks
Attachments
Comments

Confirmed for 1.13.1.

Confirmed for 1.16.3

Seems to affect 20w49a

I can also confirm the issue happening on 20w49a - when trying to make pistons facing up via flatworld preset, no matter what I try, the preset gets reset to default one.

Confirmed on 1.16.5
Can confirm in 21w07a.

Can confirm in 21w11a.

This is a feature request.

@[Mojang] Adrian Östergard
In earlier versions, you could specify block states with numbers.
Example:
minecraft:stained_glass:11
Here was "11" the metadata (today block state)
And here a old video at 3:54 you can see that meta datas/ block states was a thing:
https://www.youtube.com/watch?v=yoLzUirxNcw

I wouldn't consider this a feature request. There are some blocks that generate naturally with various blockstates such as logs, dripleaf or dripstone (see image above), and then a lot of blocks can be waterlogged. Not allowing those means you can't generate the worlds properly.
Reopened; reasoning is valid.

Can confirm in 1.20.1 with the params provided.

The second line is the syntax that's expected to work (it matches commands), so when fixing this, please use that syntax.
Affects 1.21. I'd like to request ownership of this report, as it has an inactive owner that can no longer maintain it.

Can confirm in 24w34a.
Can confirm for MC 1.12.1.