Minecraft 1.20.1
Forge: 47.2.0
Additional Placements: 1.8.0
Additional Blocks Stone Edition: 2.1.3

I'm having no problem with Placements registering stairs and slabs from minecraft, quark, stoneworks, twigs... but it won't grab the stairs and slabs from "Additional Blocks: Stone Edition."

I have tried using CraftTweaker to add the stairs to tag:blocks:minecraft:stairs with no results.

Is there something I'm missing?

If those stairs and slabs don't use the vanilla classes, the mod can't detect them. My guess is that's what's going on.
EDIT to specify: subclasses of the vanilla slab/stair classes work. Same for carpets and pressure plates.

Yep, checked their source. Custom classes. Would need a special handler.

Are you working on support for blocks that don't use the vanilla classes?

I would like to add that this is the same situation for Create: Connected's copycat stairs. They can be placed in different direction as normal, but once you start rotating or flipping them the game crashes.