iOS의 백그라운드 모드에서 FCM(Firebase Cloud Messaging) 알림이 유지되지 않는 이유는 무엇입니까?

<code class="php"><?php

$data = array(
    'message' => 'Hello World!',
    'body' => 'Hello World!'
);

$post = array(
    'registration_ids' => $ids,
    'data' => $data,
    'content_available' => true,
    'priority' => 'high',
    'notification' => $data
);

// ... Remaining code</code>

<code class="objc">// AppDelegate.h
@interface AppDelegate : UIResponder <UIApplicationDelegate, GCMReceiverDelegate>

// AppDelegate.m
- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // ...
    // Register for remote notifications
    if (floor(NSFoundationVersionNumber) <= NSFoundationVersionNumber_iOS_7_1) {
        UIRemoteNotificationType allNotificationTypes =
        (UIRemoteNotificationTypeSound | UIRemoteNotificationTypeAlert | UIRemoteNotificationTypeBadge);
        [application registerForRemoteNotificationTypes:allNotificationTypes];
    } else {
        UIUserNotificationType allNotificationTypes =
        (UIUserNotificationTypeSound | UIUserNotificationTypeAlert | UIUserNotificationTypeBadge);
        UIUserNotificationSettings *settings =
        [UIUserNotificationSettings settingsForTypes:allNotificationTypes categories:nil];
        [[UIApplication sharedApplication] registerUserNotificationSettings:settings];
        [[UIApplication sharedApplication] registerForRemoteNotifications];
    }
    // ...
}

- (void)applicationDidEnterBackground:(UIApplication *)application {
    [[GCMService sharedInstance] disconnect];
    _connectedToGCM = NO;
}

- (void)applicationDidBecomeActive:(UIApplication *)application {
    [[GCMService sharedInstance] connectWithHandler:^(NSError *error) {
        // ...
    }];
}

// ...</code>