The data value ID "stone_slab" means different things in 1.13x and the 1.14x snapshots.

• In 1.13x it is what's commonly know as the smooth stone slab, a lighter stone with dark edges.
• In the Snapshots it has the same texture as the ubiquitous regular stone, while a different block, "smooth_stone_slab" takes on the old appearance of the lighter stone with dark edges.

Both of these versions have the same pack_format number (4), but is is impossible for texture artists to texture "stone_slab" properly for both versions of Minecraft.