Android Oreo - BroadcastReceiver + JobIntentService does not work properly (GCM notifications)

Question

I have an app with GCM push notifications. Notifications are send from CMS.
My appTargetSdkVersion=27.
Everything works OK except two facts:

1. app doesn't receive notifications when it is killed on devices with Android ver lower than Oreo.

2. On Oreo I must turn on the device screen in order to receive notifications.

I have no clue why it behaves like that.
*In onReceive method of BroadcastReceiver I enqueueWork and start JobIntentService onHandleWork method to post notification.
*Before I used WakefulBroadcastReceiver + startWakefulService + IntentService and it worked fine. But I changed this implementation, because after upgrading targetSdk to 27 notifications stopped working on Oreo (I know it was caused by background service restrictions).

EDIT:
I fixed this issue by changing notification channel parameter IMPORTANCE from IMPORTANCE_DEFAULT to IMPORTANCE_HIGH!

My GcmReceiver class:

public class GcmReceiver extends BroadcastReceiver {

@Override
public void onReceive(Context context, Intent intent) {
    ComponentName comp = new ComponentName(context.getPackageName(),
            GcmNotificationService.class.getName());
    GcmNotificationService.enqueueWork(context, (intent.setComponent(comp)));
    setResultCode(Activity.RESULT_OK);
}

}

My JobIntentService class:

public class GcmNotificationService extends JobIntentService {

private static final String TAG = "GcmNotificationService";
private Random random = new Random();

static final int JOB_ID = 1000;

static void enqueueWork(Context context, Intent work) {
    enqueueWork(context, GcmNotificationService.class, JOB_ID, work);
}
@Override
protected void onHandleWork(Intent intent) {
    Bundle extras = intent.getExtras();
    GoogleCloudMessaging gcm = GoogleCloudMessaging.getInstance(this);

    String messageType = gcm.getMessageType(intent);

    if (!extras.isEmpty()) {
        if (GoogleCloudMessaging.MESSAGE_TYPE_SEND_ERROR.equals(messageType)) {
            Log.e(TAG, "Send error: " + extras.toString());
        } else if (GoogleCloudMessaging.MESSAGE_TYPE_DELETED.equals(messageType)) {
            Log.e(TAG, "Deleted messages on server: " + extras.toString());
        } else if (GoogleCloudMessaging.MESSAGE_TYPE_MESSAGE.equals(messageType)) {
            String title = (String) extras.get(Config.MESSAGE_TITLE);
            String description = (String) extras.get(Config.MESSAGE_TEXT);
            boolean isUpdate = false;
            try {
                isUpdate = 1 == Integer.parseInt((String) extras.get(Config.MESSAGE_UPDATE));
            } catch (NullPointerException | NumberFormatException e) {
                e.printStackTrace();
            }

            if (new Preferences(this).getBool(Constant.PREFS_NOTIFICATIONS_ENABLED, true)) {
                sendNotification(title, description, isUpdate);
            }
        }
    }
}