From what I can gather, it is just one of many Pokémon which lacks a backsprite for one of its alternate forms, shinies included. Quite a few other Pokémon with gender differences have only one backsprite, but go unnoticed for a lack of obvious differences.Posting this visual bug (I guess) for the user: Flamemaster5000.
In this battle: https://replay.pokemonshowdown.com/gen7randomdoublesbattle-701517907, he has a female unfezant, as you can see from this screenshot: View attachment 101055
However, its backsprite shows a male unfezant: View attachment 101056
Is this just a visual bug? Thanks in advance!
Trivia: For some reason Meowstic-F has two backsprites but only one frontsprite: https://play.pokemonshowdown.com/sprites/xyani-back/meowstic-female.gif and https://play.pokemonshowdown.com/sprites/xyani-back/meowstic-f.gif. The same is true for Rotom-Frost, which also has one under Rotom-F. (All Rotom formes have 2 frontsprites)