What I expected to happen was...:

When breaking a double resin slab block with any level of pickaxe 2 slab items should have appeared

What actually happened was...:

When breaking the double slab block only one slab item appeared

Steps to Reproduce:

1. Place down two resin slabs on top of one another to make a full block
2. Using any level of pick, break the block and only one appears instead of 2

Code analysis (official mappings)

In the VanillaBlockLoot#generate method:

this.dropSelf(Blocks.RESIN_BRICK_SLAB);

instead of

this.add(Blocks.RESIN_BRICK_SLAB, $$1x -> this.createSlabItemTable($$1x));

Code analysis (Yarn mappings)

In the VanillaBlockLootTableGenerator#generate method:

this.addDrop(Blocks.RESIN_BRICK_SLAB);

instead of

this.addDrop(Blocks.RESIN_BRICK_SLAB, block -> this.slabDrops(block));