Environment:
Language: NodeJS
Server: Node Express
Device: Huawei P20, iPhone 11

Description
When using liff.sendMessages() to send flex message on behalf of a user, the app context event is not being obtained

To Reproduce

1. Open liff app
2. Call liff.sendMessages() to send a flex message on behalf of user. For instance:

liff.sendMessages([
  {
    "type": "flex",
    "altText": "Game mode",
    "contents": {
      "type": "bubble",
      "body": {
        "type": "box",
        "layout": "vertical",
        "contents": [
          {
            "type": "text",
            "text": "Player 1 selected",
            "weight": "regular",
            "size": "md",
            "align": "center"
          },
          {
            "type": "text",
            "text": "C01",
            "weight": "bold",
            "size": "3xl",
            "align": "center",
            "offsetTop": "xxl"
          }
        ],
        "height": "130px"
      }
    }
  }        
]);

3. Print context at the backend

Expected behavior
App context event should be captured when sending flex message via liff.sendMessages(). According to the LINE docs, webhook events contain the contentProvider.type property = external parameter when image message (flex) is sent using the liff.sendMessages() method, but it seems like this event is not being handled in the codes?