CoreData relationships are nil after mergeChangesFromContextDidSaveNotification

Having some truly strange behavior with Core Data on iOS. I have a NSManagedObjectContext for my main thread used to read data from a SQLLite persistent store and display it to the user. I also have background processes managed by an NSOperationQueue. These background processes create an NSMangedObjectContext, fetch data from remote servers, and persist that data to the local Core Data store. I have registered for NSManagedObjectContextDidSaveNotification and when I receive those notifications call mergeChangesFromContextDidSaveNotification on the main threads NSManagedObjectContext (passing the notification object as an argument).