Hey!
I just discovered a bug!
I've sent a battle request to another player and changed my 1st Pokemon just in the moment, the battle began.
But the pokemon, that was at place one at first, was still sent into the battle at first, but in the list of my team on the left side another pokemon was 1st place now.
The pokemon in battle (not 1st in list anymore, but sent into battle as 1st) fainted and the Pokemon which was 1st in list also fainted, but it was changed into another pokemon before the battle has started.
The pokemon, which was in battle, was still chooseable but when i chose it, i couldn't do any actrion with it and had to change.
The pokemon, which was in battle, was then displayed as already fainted and the pokemon, that was exchanged for it, before the battle, was alive again.
So I would say that if you switch the 1st place of your team, just in the moment a battle starts, it could (?) lead to unwanted behaviour.
I hope I described the events the right way and in the right order.
Greetings!