In ePO 5.9, you have a tag whose criteria for assignment uses the contains pattern comparison. If this tag is configured to be evaluated on each agent-server communication, it might not be correctly assigned.

If you run the tag criteria manually, or via a server task, the tag is correctly assigned to matching systems. But, the next time a matching system communicates with ePO, the evaluation is not successful, and the tag is removed from the system.