Last night I searched alone in Dominion (so it’s partly my mistake I suppose) and joined a game in progress, which is usually fine - except the game put me on the losing team, which was in last stand (the enemy team had captured all of the bases) with only one other player alive. Without team support, I wasn’t able to re-capture a base by myself.
The window for join-in-progress in Dominion games should close when a team puts the other into last stand. Inevitably, players will quit if they can’t respawn, leaving spots open for new players to be inserted into a losing situation. In addition, it’s not fair for the winning team if last stand is perpetuated by people quitting out and join-in-progress filling in a new player. Of course, the window for JiP can be opened again if the team in last stand recaptures a base.