The game auto-removes the 4th party member for the new one. Download latest version of the game to fix this issue. Just in case enter the battle with a party of 3 to avoid glitch.
I did, that's the issue, although... they were greyed out and didn't exactly show they were in my party, maybe they were still considered in? I don't know...
But the fact that Biyo was able to join and me going with hackmon and it worked makes it more confusing...
