Mx is gender neutral and is just as likely used because someone doesn’t want to use any gender. I wouldn’t focus on the individual. We don’t usually want gender to be at the core of everything and most certainly would prefer work to one of those places where it shouldn’t matter. My suggestion would be to just focus on the fact that the HR systems do not handle Mx at this time even though you are fully supportive in matters of diversity and that the issue has been escalated. Hopefully you’ll have an answer from HR before you have to have the conversation. You may want to be delving into the area of diversity training for your team too with HR.