Tested by hammering a double Crag Stone Slab from Traverse. I assume since there's no entry for the double slab as an item (at least it can't be found in JEI), it produces no tiles for your bag.

Expected behaviour: when a block like this is found not to have any tiles, produce tiles of the slab instead?

It's fixed in the newest pre-release. Slabs are now excluded entirely. Thanks for reporting this issue!

You can't hammer a slab, same for stairs, fences, farmland, etc. Only full blocks are acceptable for LT.

Oops. Still kinda sucks that it doesn't give anything back.

Technically that is not supposed to happen. Can you replicate this behavior with vanilla double slabs, such as stone or wooden planks?

Well that's... something

Does it still break the slabs or does it just give air?

When hammering double slabs, it breaks down and I get tiles of those blocks called "Air." They still place like normal, but only by taking the tiles out of the bag, the chisel won't work.

Does it still happen in pre162?

Ok, so now when I hammer double slabs, it still works, but I get no tiles. In fact, I get no tiles from anything.