Being a tank sitting next to a dps has permitted me to test the theory that the people in the group that had someone withdraw are put at the front of the line is incorrect.

Easiest way to test this is using Main Scenario DR, have the dps queue then when they are placed in reservation there is always at least one tank still needed. At this point the tank queues getting placed into that same group. Someone then either withdraws or misses the queue. At this point the tank is immediately placed in another group but the dps has another 30-45 minute wait as they were sent to the back of the dps line.