The sprites for the 7th Generation pokemon aren't shiny from the back if you pick a shiny pokemon (for example, if you have a shiny Palossand, it will still look normal from behind, but your opponent will see the shiny Palossand)
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Also, if you re using an Unfezant, it looks like the male sprite from the back, even if you have a female.
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Most of the missing backsprites aren't being added for a simple reason: nobody has made them yet. They will of course be added whenever someone makes them but until then we're using the simple solution aka using the closer sprite possible