Alrighty, I ran that with just a few slabs in my inventory and I get stuff like this:
[{Slot: 0b , id: "minecraft:stone_brick_slab", tag: {}, Count: 9b}, {Slot: 2b , id: "minecraft:stone_brick_slab" Count: 1b}]
It looks like some items lack a tag field? which would explain why they wouldn't stack with those that do have a tag field.
Any ideas on what to do with this info?