If your site you have a support agent or manager for example
you are the administrator and you try to assign a ticket to the support manager or agent
When you select the ticket and looking at the list of support staff to assign to another person only see the name of the administrator to assign no other agents
If you go to Advanced and check the box for the support agent can view the ticket this sees it but appears only assigned to the administration because you can not change
If you assign a support agent in the old version 3.1.9 and upgrade to 3.1.11 assigned appears in the current version, but if you respond to a ticket assigned automatically lose of assign and this is assigned to the administrator
This began to happen in version 3.1.11 on older versions not have this problem.