I just acked an alert on the dashboard named '#75664: warning: hardware: on co-back03' under the 'warning - {component=Main_System_Chassis}' category. I would expect this to change the last action displayed in the header, but instead it still shows:

If I look at the actions under http://bosun.ds.stackexchange.com/incident?id=75664 it only shows my acknowledgment (no close actions from Kyle).