imUrlDataApp Send from App displays weird controller rendering of message

The documentation for setting the `message` parameter in Apple's [MFMessageComposeViewController doc][m1] says:

> If your app has an iMessage app extension, you can display your iMessage app within the message compose view, just as you would in the Messages app. To display your iMessage app, create and assign an [MSMessage](https://developer.apple.com/documentation/Messages/MSMessage) object to this property.

That _works_ to enable sending a custom message, that will be properly directed to the matching iMessage extension, as seen in imUrlDataApp

However, as seen in the screenshot below, the message dialog doesn't display our custom controller's rendering. Instead there's a weird little lozenge with a copy of our icon and a cross. Tapping the cross removes our attached `message` property and reduces the message to a simple text message.


[m1]: https://developer.apple.com/documentation/messageui/mfmessagecomposeviewcontroller/message