I have just contacted Facebook support and their response
“Thank you. So far, we’ve confirmed that this is an issue with how the SSL is setup. The technicality should be supported directly by your Web developers or domain provider. The URL may not be served properly on mobile phones which has different system base.
You can share this tool with your developers : https://developers.facebook.com/tools/debug/
The URL is being redirected to the unsecured version (http) before going to back to the original URL.”
Is your solution still valid?