Android Device cannot registered with Urban Airship

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We recently noticed that no device was registered at all with Urban Airship. When debugging, we noticed that the following two apis are never hit now. When it was working, we can see these apis were hit and channel id was created. 

public class AirshipListener implements PushListener, NotificationListener, PushTokenListener, AirshipChannelListener {

    ...

    @Override
    public void onChannelCreated(@NonNull String channelId) {
        //some code not shown here
    }

    @Override
    public void onChannelUpdated(@NonNull String channelId) {
        //some code not shown here
    }

    ...

}

In build.gradle (app), we have these line:

    // Note : All Airship dependencies included in the build.gradle file should all specify the exact same version.
    def airshipVersion = "13.3.5"
    implementation "com.urbanairship.android:urbanairship-core:$airshipVersion"
    implementation "com.urbanairship.android:urbanairship-fcm:$airshipVersion"

When debugging, we do see credentials are there and passed to the following method:

public AirshipConfigOptions createAirshipConfigOptions(Context context) {
            AirshipConfigOptions options = new AirshipConfigOptions.Builder()
                    .setDevelopmentAppKey(MySharedPreferences.getUAAppKeyDev())
                    .setDevelopmentAppSecret(MySharedPreferences.getUAAppSecretDev())
                    .setProductionAppKey(MySharedPreferences.getUAAppKeyProd())
                    .setProductionAppSecret(MySharedPreferences.getUAAppSecretProd())
                    .setInProduction(BuildConfig.UA_IS_PROD)
                    .setFcmSenderId(BuildConfig.GOOGLE_PROJECT_NUMBER) // FCM/GCM sender ID
                    .setNotificationIcon(R.drawable.ic_notification)
                    .setNotificationAccentColor(ContextCompat.getColor(context, R.color.abc_blue))
                    .build();

            return options;
    }

We haven't really make any code changes recently except upgrade "airshipVersion" in build.gradle, but I tried to rollback to previous value, still registration with Urban Airship fails. So where comes this issue all of sudden? Please help!

Thanks in advance!

P..S, not sure which Topic to select so I leave it not selected at all.