We have found an issue that assigns a group to "Not Assigned" if the Technician is not in a support group and even if "Group" is a required field. See the example below.

Tech1 is assigned to SupportGroup1 and SupportGroup2. A ticket is sent in and is assigned to SupportGroup3. If Tech1 opens the ticket that is assigned to SupportGroup3 and selects "Assign --> Pick Up", then the ticket will be assigned to Tech1 and the Support Group will be set as "Not Assigned"

SDP 9300.

I had a similar issue that was supposed to be patched soon.

https://forums.manageengine.com/topic/group-can-be-set-to-none#49000008333846