With using heat template for clean-up, it's been observed that when the interface is removed from router north-bound event is received, the ietf-interface entry in config DS been already deleted. As a result, the router-interface remove event returns with only deletion of interface-to-router mapping and not deleting any entry from neutron-router-dpns(as unable to obtained dpn for the interface). 

When interface-state south bound event is triggered, interface-to-router mapping is already deleted and this event also returns without deleting entry from neutron-router-dpns. As a result, the earlier continues to act as napt switch for a given router even through it doesn't has any router presence.