Once again, Google’s John Mueller said on Twitter that websites should not use m-dot domains (m.example.com) for mobile websites. Google prefers responsive websites that work with desktop computers and mobile devices.
My 2 cents on this — if you have this situation (m-dot robotted, desktop ok), I wouldn't spend too much time trying to analyze it, I'd just fix it. Robot both or robot none. Also, long-term, move from m-dot, it's an unnecessary time-sink. Make life easy, focus on what matters.— 🍌 John 🍌 (@JohnMu) November 12, 2019