Just like the old version, if the creator doesn't close or end the fireteam finder and other people join without specifically using it so it auto-closes, then it'll remain open. The previous version of the Fireteam Finder had the same issue. I didn't use it much, so I'm not sure if Bungie ever made them time-limited on the old version of the Fireteam Finder, but hopefully they will on the new version. I'd suggest heading over to the Destiny 2 - Feedback section if you want to leave your feedback about old ones sticking around too long.