When an agent updates a ticket's Assignee field and submits the changes, the ticket keeps automatically reassigning to the same agent or agent group upon submitting the ticket.
Cause and resolution
In many instances, this issue is a result of a trigger that meets the condition of the ticket, that is then making the change automatically upon submitting the ticket to save the changes.
To pinpoint the trigger likely causing the reassignment, click Show all events and check for the triggers that fired on the ticket during the latest update.
You will then need to either change the properties of the ticket to not meet the conditions of the trigger or update the conditions of the trigger.