CloudKit reference subscription requires CKReferenceActionDeleteSelf reference?

I've created a subscription for child record changes, based on a reference field to a parent record.

When I create a child record and set the child's parent reference with a CKReferenceActionNonereference, the subscription does not generate a push notification. If I use a CKReferenceActionDeleteSelf reference, the subscription works as expected, generating push notifications on all subscribed devices.

I do not want want cascading deletes. Why do I need to use a CKReferenceActionDeleteSelfreference to generate push notifications?

Here is the code:

    // subscription registration
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%K == %@", @"parent_ref", _parentRecordID];
    CKSubscription *subscription = [[CKSubscription alloc] initWithRecordType:@"Child"
                                                                    predicate:predicate
                                                                      options: CKSubscriptionOptionsFiresOnRecordCreation|CKSubscriptionOptionsFiresOnRecordUpdate|CKSubscriptionOptionsFiresOnRecordDeletion];
    CKNotificationInfo *notificationInfo = [[CKNotificationInfo alloc] init];
    notificationInfo.alertBody = @"Alert body";
    notificationInfo.shouldBadge = YES;
    notificationInfo.shouldSendContentAvailable = YES;
    subscription.notificationInfo = notificationInfo;
    [_db saveSubscription:subscription
        completionHandler:^(CKSubscription *subscription, NSError *error) {
            // completes without error
    }];

    // child record construction
    CKReference *parentRef = [[CKReference alloc] initWithRecordID:_parentRecordID
                                                            action:CKReferenceActionNone];
    CKRecord *childRecord = [[CKRecord alloc] initWithRecordType:@"Child"];
    childRecord[@"parent_ref"] = parentRef;
    [_db saveRecord:childRecord completionHandler:^(CKRecord *record, NSError *error) {
        // completes without error, but does NOT generate a push notification.
        // HOWEVER, if parentRef.action is CKReferenceActionDeleteSelf instead of
        //      CKReferenceActionNone, then push notifications fire on other devices,
        //      as expected.
    }];

The -saveRecord: method completes without error, but does NOT generate a push notification. HOWEVER, if parentRef.action is CKReferenceActionDeleteSelf instead of CKReferenceActionNone, then push notifications fire on other devices, as expected.

Why would this be?