Have you try to run a Full Sync Scan on this device ? (make sure that the inventory is updated and the scan not rejected on the Core server). Then refresh the Security Activity pan and double check if the the computer is still showing up in the two sections.
Have you installed any CPs (component patches) on top of your 9.6 SP2 ?
Lastly, we have released the Service Pack 3 couple of days ago, more information here: