Niloufar Ahmadinikoo

We are facing a critical issue. We manually change agent groups often for the purpose of training. Yesterday we learned that each time an administrator changes an agents' groups, all tickets that were solved by an agent in the group they are no longer in, gets automatically assigned to the administrator updated the group assignments. This is happening for all tickets regardless of status AND is NOT happening through a trigger. 

This is causing a mass issue in our system because we have >1800 tickets that are now assigned to someone other than the original agent who solved the ticket.

Is this a bug? What can we do to automatically route these tickets back? How can we ensure this does not happen again?