I disbanded a team (which had only me in it) during MP and got an immediate Defeat. This probably won't happen often (I only did it because I noticed the chat was still open), but it might be easy to fix? Just to be clear, I was the only one in the "team".
I noticed that whenever you close the chat for your team, it basically boots you out of whatever you're in. That does make reasonable sense, though, so the real problem could be that it's letting you enter things it shouldn't while you're still in team.
That just makes it more confusing for me. Yesterday I had an idea to try and solo-grind campaign using the extra draw cards from duo-co-op. So I got someone to join a team, then exit, leaving me in an empty team. I then tried to enter a mission, sure I would get my extra draw... nope. So, it seems that when solo (but with old team chat open) "being in a team" only applies to the "exit on disband" part of team membership. Perhaps one solution would be to completely remove forced exits/defeat from "disband team"? I won't pretend to understand the complexity of the programming, but I like this scenario for team quits/disbandment: Member quits... member dies. Member continues to "auto-die" at the beginning of each sub-mission, but team is allowed to continue said mission (with an obvious handicap). Team leader quits (with remaining members) team leader dies as above and leadership switches to... whoever. Empty teams are auto-disbanded at the end of the current mission. Yeah, that must be pretty hard to actually make happen, cause it seems like the devs really care about their game, and it hasn't happened yet. Anyway, it will definitely be a case of once bitten twice shy, so at most we just have every forum member posting here once... seems like not so much of a problem.
Thanks for testing & reporting this @myothername . Drawing an extra card would have made solo runs easier, but I doubt it's what the devs intended. So when I get the solo-bug, I'll have to keep doing it the old fashioned way. It would have been nice for a solo nudie run in particular. My attempts so far have all ended fairly quickly.