I could be wrong but if I remember correctly the EGR should be closed at idle. I remember when I worked at the dealership diagnosing vehicles with a rough idle by using a screwdriver on the stem to pry open the egr valve and then letting it snap shut. Sometimes this would unstick a stuck valve or break up carbon in the seat of the valve causing it to seal back up.
If the plate is only sealed around the perimeter it could leak across the dividing wall. It doesn't take much for a vacuum leak especially if there is pressure on the exhaust side.
