Dungeon Problem - Am I missing something?

[MrThinker]

Whats probably happened is if you waited about 30 seconds longer the replacement would have shown up, and because the replacement was about to load in, it didn't treat it as a partially empty party

I can confirm this is a thing that happens. How long do you usually wait for a replacement? Generally speaking, you should wait around 5-6 minutes for another player to show. After that it becomes increasingly unlikely that a player will zone in, and its up to you if you want to stay or not. I usually wait 10 minutes myself before calling quits.