UPDATE 2020-10-09: A resource pack designed to fix this issue specifically for polished granite slabs has been added. Mojang are free (and encouraged) to use these assets or derivatives thereof to fix the issue in question, or if desired use these as a reference point for the fix.

The bug

The sides of several slab blocks do not have the shading on the top/bottom to indicate they're actually a slab, resulting in the texture instead abruptly cutting off.

The following "polished" slab blocks are affected:

• minecraft:cut_red_sandstone_slab
• minecraft:cut_sandstone_slab
• minecraft:polished_andesite_slab
• minecraft:polished_blackstone_slab
• minecraft:polished_deepslate_slab
• minecraft:polished_diorite_slab
• minecraft:polished_granite_slab
• minecraft:quartz_slab

The stair forms are similarly affected (see MC-262651).

Note how regular smooth stone slabs are not affected by this, having a lighter appearance at this spot and appearing correctly polished.

A partially related thing of note is that normal sandstone and red sandstone bottom slabs, while not "polished", also lack the few pixels at the top of normal sandstone blocks.