Problem with FCM notifications when device offline

I have a problem that I have not been able to solve. I currently work with ionic and vue to make an android app. The problem appear when I want to send a push notification through a curl request when the device is not connected to network (offline). If I send a single notification is no problem but when I send more than one is always overwritten and only the last one arrives to the mobile.

When the mobile is connected to the internet, all notifications arrive without problems.

I tried to setting the time_to_live parameter but apparently doesn't make difference.

Here is the code:

function sendNotification($i){
    $apiKey = "API_KEY";
    $tokens = ["CLIENT_TOKEN"];
    $data   = array(
        "registration_ids"  => $tokens,
        "notification"      => array(
            "title"         => "title ".$i,
            "body"          => "content ".$i,
            "color"         => "#33DDFF",
            "priority"      => "high",
            "collapse_key"  => 123
        ),
    );

    $headers = array(
        "Authorization: key=".$apiKey ,
        "Content-Type: application/json"
    );

    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL            , "https://fcm.googleapis.com/fcm/send");
    curl_setopt($ch, CURLOPT_POST           , true);
    curl_setopt($ch, CURLOPT_HTTPHEADER     , $headers);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER , true);
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER , false);
    curl_setopt($ch, CURLOPT_POSTFIELDS     , json_encode($data) );
    $result = curl_exec($ch);
    curl_close($ch);
}

for($i=11;$i<16;$i++){
    sendNotification($i);
}