A bug in Google caused the show desktop URL's of sites in mobile results
Recently English-speaking users have noticed that Google started showing the desktop URL in mobile search results.
Some industry experts have speculated that Google has begun to launch mobile-first index. However, later it became known that the cause of the show desktop URL was the bug in the search engines.
A Google spokesperson John Mueller commented on the situation on Twitter:
yeah, looks weird :). We noticed too and are looking into it on our side. Seems like a quirk on our side.
Previously in November of 2016, Google began testing mobile-first index. This means that in the near future the algorithms of search engines will first evaluate the relevance of the mobile version of the page and in its absence – the desktop.
The FAQs on this change can be found here.