if you ack issues on dashboard this make him "acked", but if you go to triggers view you will see this trigger unacknowledged.

but if you ack trigger in trigger view then you will see also acknowledged on dashboard.

i play with this and findout that if you ack thru triggers view - you ack all current and prev events at all. if you ack on dashboard you only ack last one event. but i still dunno why triggers looks unacknowledged while ack em thru dashboard.