While I've never experienced anyone intentionally leaving the team up or sigil right before it counts down to zero, I believe there should be a mechanic to increase the timer whenever someone leaves at any point during the time. To avoid griefing, this would only happen once. (This is to avoid infinite time before the timer hits 0)
For example, let's say you're teaming up for a Waterworks run. There are currently 3 players in the same team up, and the timer is about to reach zero. One of the players decides to ruin the fun and attempt to leave right before the timer ends. However, with this new addition, the timer would increase +30 seconds, allowing more players to join, but this would only happen once. So if the player instantly rejoins the same team up, and tries to increase the timer by leaving, it would now obey the old rules and continue counting down.
For sigils, it's the same deal, except the timer just resets back to 10, and then begin counting down back to 0 like normal.
The purpose of this is mainly to let the players actually trying to enter a dungeon have time to react to the sudden decrease in player count queued for the dungeon.
Again, I have never experienced either of these, so I considered this idea as "Something nice to have," rather than a complaint.