Slabs don't let redstone signal pass through about full-slabs

[Micalobia]

This is actually a feature in Vanilla, putting a lever on a top slab doesn't power the slabs next to it, and the same with bottom slabs. This actually happens with stairs as well, and every non-opaque block in the game, despite being able to put levers and buttons on them.

TLDR; It's not an opaque block so it can't be powered in general, and it shares this property with stairs which feels even weirder.

[SplendidAlakey]

Oh, shame. I didn't think it was vanilla "feature" because I can't put levers on horizontal slabs at all.

[Micalobia]

Yeah, I agree with ya, feels a bit weird that it behaves like that. Patching it would be both way outside of the scope of this mod, and just generally break a lot of redstone, so it's generally just a bad idea even though it'd be neat.