Google’s John Mueller has confirmed on Twitter that Google does not recommend separate mobile URLs. Google recommends responsive web design that uses a single URL for desktop pages and mobile pages. Separate URLs for the same content make things harder than necessary.
Looks like the mobile URL isn't being picked up as belonging to the desktop URL. That can sometimes happen, and is one of the reasons we don't recommend separate mobile URLs anymore. Separate URLs for the same content makes things unnecessarily harder.— 🍌 John 🍌 (@JohnMu) June 20, 2019