The sides of quartz, cut sandstone and red sandstone, and polished granite, diorite and andesite slabs appear cut off / double versions of these slabs look like normal blocks

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 quartz, cut sandstone, cut red sandstone, polished granite, polished diorite and polished andesite slabs do not have the shading on the top (if bottom half)/bottom (if top half) that gives the full polished blocks their polished appearance, instead abruptly cutting off where the block ends.

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

The stair forms are similarly affected.

Similarly, bottom-half sandstone and red sandstone slabs don't have the same sort of top border on them like the top-half and full block variants.