It seems the new recipe unlocking system can't tell what type of slab is needed in order to craft the armor stand. This also seems to happen with other crafting recipes that include a specific block data value. For example, smooth sandstone unlocks the chiseled sandstone recipe, even though you cannot craft smooth sandstone with chiseled sandstone.
MC-115033
Recipes for certain item subtype are unlocked for any subtype of item
Resolved
Fixed
2
When Grum's block ID/state system revamp is implemented, this should hopefully be fixed, since the different slab types will become separate block IDs.