The Group status is based on the worst possible status of any object associated with the group. In your case the should be some events with severity Error associated with one of the 2 filers, but that event may have been deleted. This may be a reason why when you go to events page you see nothing. Though the event has been deleted, but the condition because of which the event was triggered hasn't changed the status of the object remains the same. Because of which you are encountering what you described. Check the events-history or event-deleted report to see if there are events associated with those 2 controllers and event condition remains the same.
BTW the group status is a visualization to tell you some thing that belongs to the group is being affected. If you are already attending to individual events you shouldn't care much about group status. In fact its practically impossible to have the group status as green unless your group is empty
There are 20 other controllers under DFM and if I see that in the Event history there are multiple errors/warnings/emergencies logged against those controllers as well. Why am I not seeing anything like this on those controllers? Is there a Bug etc.. because of which you are saying what you are saying.
I have never seen Ops mgr behaving like this in the past. But then again, that does not mean that it can't happen.
And yes the team monitors the Alerts periodically but as I said "It's just the color of the group which is bothering me. Otherwise, everything else works fine."