Oh what's that, you finally got a full party and reserved an instance but someone in that queue left? Let's disband the whole party and have the server find them all over again from scratch and wait for the server to create a whole new instance instead of saving both the existing players that didn't leave the queue and the instance that was already reserved and just finding a new player to fill that slot.

Pretty sure that's how the duty finder works most of the time or else queues would be much faster after someone leaves a spot.