When the quartz slab got its own ID, its block definitions got ported incorrectly.

The quartz block was changed years ago to not use the borderless texture anywhere, but its texture file is still deceptively named. This can be fixed by changing the quartz_slab_bottom definition in terrain_texture.json from this...

    "quartz_slab_bottom": {
      "textures": "textures/blocks/quartz_block_bottom"
    }

...to this:

    "quartz_slab_bottom": {
      "textures": "textures/blocks/quartz_block_top"
    }

This would make quartz slabs match quartz blocks once again.