Yes i was right. Joomla natively has nothing doing this kind of https force, i think that there is a third party plugin of type 'System' installed that do a brute replacing of every links that Joomla outputs with 'https' prefix.

You can easily test it opening the XML sitemap document, the first line must have everything in 'http' and not 'https' as visible below.

As a final though, there could be also a system on the server doing this. But in this case you should contact the hosting provider, the problem would be outside Joomla.