When closing a ticket, the team/agent assignment is removed. Agents with 'assigned only' access can no longer see it.

This is by design. Closing a ticket removes all assignments. 'Assigned only' agents only see open assigned tickets. To give agents visibility into closed tickets they worked on, use Department-based access instead -- agents with Department access retain visibility to all tickets in that Department regardless of status or assignment. Enable Re-Open Auto-Assignment if you want the closing agent to be re-assigned when a ticket re-opens.