I like me some dungeons, so I run them often. However, regularly, when doing a dungeon via Dungeon Finder (whether random or specific), it drops the queue after you have accepted your role.

For an example, today I'm attempting City of Ash 2, and keep getting dropped at the entry phase. Here's a screenshot showing I accepted the queue (I'm the healer).

https://i.imgur.com/lMGVDYP.png