Facebook Messenger Extensions on Microsoft Edge -- GET parameters absent

I want to use Facebook Messenger Extensions (opening a Webview) with Microsoft Edge.

My app sends a request to the REST API endpoint messages. The payload is as follows. Documentation on this.

{  
   "messaging_type":"RESPONSE",
   "recipient":{  
      "id":"<id>"
   },
   "message":{  
      "attachment":{  
         "type":"template",
         "payload":{  
            "template_type":"button",
            "text":"Test text",
            "buttons":[  
               {  
                  "type":"web_url",
                  "url":"https://c0292fc0.ngrok.io/facebook/render_multi_choice?data=24561",
                  "title":"Test Button",
                  "webview_height_ratio":"full",
                  "messenger_extensions":"true",
                  "fallback_url":"https://c0292fc0.ngrok.io/facebook/render_multi_choice?data=24561"
               }
            ]
         }
      }
   }
}

When the user taps the button "Test Button" in Messenger, a Webview pops up and displays the response at the fallback URL / URL : https://c0292fc0.ngrok.io/facebook/render_multi_choice?data=24561

Note the URL contains one parameter, data = 24561

My Python Flask App receives the request:

@application.route("/facebook/render_multi_choice", methods=["GET"])
    def render_multi_choice():
        args = request.args
        print(args)
        return "OK"

The issue is that one would expect args to be printed out as:

ImmutableMultiDict([('data', 24561), ('fb_iframe_origin', '')])

Yet, it is printed out as:

ImmutableMultiDict([('data', ''), ('fb_iframe_origin', '')])

My flow works fine in Google Chrome, but NOT on Microsoft Edge. Do you have any idea what the issue could be? If I open the URL directly in the browser (without the Webview), things work as expected on Edge.