My organization has a handful of support queues based on the type of issue received. A new queue was recently established for a new area of support. In the new LibRel queue, when a support staff member initially opens the (email to case created) case to assign to the proper account and staff member, saves the changes - the case is no longer visible in that queue. The staff member may see the case in their open cases; however the case is no longer displayed in the LibRel queue.
A Queue is an owner of a case. As soon as the case is reassigned, then the queue no longer owns the case so it would make sense that it's no longer visible in that view. From the documentation (emphasis added):
Queues help your teams manage leads, cases, service contracts, and custom objects. Once records are placed in a queue manually or through an automatic case or lead assignment rule, records remain there until they're assigned to a user or taken by one of the queue members. Any queue member or users above them in the role hierarchy can take ownership of records in a queue.