We’re aware of the problem. It’s not “ran out of powerups” though. If you queue for an online match while playing boss rush, after the match you get an extra powerup. At that moment, all bets are off. That’s the bug, you’re now in a broken state, and nothing else after that point matters to report.
We have fixed the bug internally and will be in the next update. In the meantime, know that if you queue for online matches while playing boss rush, that boss rush will be broken in this way, unfortunately.