Happened again shortly after. Not sure how I triggered it, but it allowed me to enter a 3 energy fight with no energy by tapping the training menu and then the team selection button.
I then proceeded to die to Model Leader like a true chad :')
Found out how to reliably trigger this weird state:
Step 1) Lose or win a match when one of your characters would not have enough energy to retry
Step 2) hit retry. When prompted to use energy refills, hit change team
Step 3) hit either back a menu or home
Step 4) hit the team change button and hit fight to enter the same fight.
This needs to be patched, this allows players to enter fights even if their characters are out of energy.
Characters can be changed before the fight, so one can purpoesfully lose to let in other characters.
I kinda have the same thing too, but instead just stacks of characters in the back, the cause might be because multiple people logged in on one account but idk.