The textures used for blocks of quartz and smooth quartz have names that do not reflect their usage accurately as of the latest version. Upon their introduction in 13w02a, their names did in fact reflect the places in which they were used (file names have changed since, however their general implications remain the same), however changes in relevant block models have rendered such naming incorrect.

History - why these are named the way they are

Originally, blocks of quartz had dedicated top, side and bottom textures, reflected in the textures being named "quartz_block_top.png", "...side.png" and "...bottom.png", respectively. The top and side textures have always been visually identical (except for 18w43a-19w13a, in which they differed by a single pixel).

The original (likely accidental) addition of smooth quartz resulted from the reimplementation of what would later become smooth stone. 17w47a solidified smooth quartz as an intended block, with its only visual distinction from the normal quartz block being that it used the top texture on all sides, resulting in the bottom texture being the only visually different face in the default texture pack. This lack of distinction was resolved in 19w08a as MC-137722, with smooth quartz then being made to use the quartz block's bottom face on all sides.

19w09a made a further change to resolve MC-144517, making the block of quartz use its top texture on the bottom. As a result, from 19w09a onwards, the "quartz_block_bottom.png" texture file has no longer been used as the bottom texture of the normal quartz block. This was further reinforced in 20w10a with the fixing of MC-152441, where normal quartz stairs would no longer use the texture either.

As a result of these changes, the texture called "quartz_block_bottom" no longer has an accurate name at all.

Suggested fix

Fixing this would be as simple as renaming "quartz_block_bottom.png" to "smooth_quartz_block.png" and repointing all associated models.

Other thoughts

See MC-226324 - Is it really necessary for normal quartz blocks to have two textures which are visually identical anyway? It'd probably be more efficient to just use a single "quartz_block.png" instead, rather than wasting space with two identical files.