The bottom texture for minecraft:quartz_stairs when as a concave or convex corner (i.e. not the default stair shape) uses the texture for minecraft:smooth_quartz.
REPRODUCE:
1. place minecraft:quartz_stairs block inverted.
2. place minecraft:quartz_stairs block inverted to the right of first block, facing an orthonogal direction
3. view difference in texture
Based on MC-152441, this is fixed for 1.16, however, the fix may not extend to the programmer art.