Updated information using NSManagedObjectContextDidSaveNotification rollback on applying rollback on Context

Question

I am using CoreData with multiple contexts (Not Parent-Child). So, I am using NSManagedObjectContextDidSaveNotification observer to update the information in different contexts. One wired issue I am facing is when a saved data on a background thread is updated on the main thread using NSManagedObjectContextDidSaveNotification then if in future I apply rollback on main thread context then it rolls back the updated info.

Can anyone explain why it is happening?

NSMergePolicy *policy = [[NSMergePolicy alloc] initWithMergeType:NSMergeByPropertyStoreTrumpMergePolicyType];
    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if ([NSThread isMainThread]) {// main thread context
        if (!self.managedObjectContextMainThread) {
            self.managedObjectContextMainThread = [[NSManagedObjectContext alloc] init];
            [self.managedObjectContextMainThread setMergePolicy:policy];
            [self.managedObjectContextMainThread setPersistentStoreCoordinator:coordinator];
        }
    }else{
        context = [[NSManagedObjectContext alloc] init];
        [context setPersistentStoreCoordinator:coordinator];
        [context setMergePolicy:policy];
    }