I think I may have found a bug with this. Trades that involve two expanded Pokémon will display the Pokémon you are receiving with the palette of the Pokémon you are sending. This doesn't occur if only one member of the trade is an expanded mon, though.
For example, if the trade is my Turtwig for their Chimecho, it works:
And, if the trade is my Chimecheo for their Turtwig, it still works:
However, if it becomes my Turtwig for their Chimchar (or any expanded mon), this happens:
You can, of course, replace Turtwig or Chimchar with any expanded mon or Chimecho with any regular mon and the same will happen. I assume this is just the case of an unedited limiter, but I've not really got any idea of how to find it.