Oauth/userinfo isn't returning BattleTag

We’ve built a login with battle.net integration but have found one of our users doesn’t have a battle tag returned when we request the user profile. What we expected when hitting https://eu.battle.net/oauth/userinfo was this:

{
  "sub": "4xxxxxxx5",
  "id": 4xxxxxxx5,
  "battletag": "FoxyBlue#21761"
}

But in this case we only see:

{
  "sub": "4xxxxxxx5",
  "id": 4xxxxxxx5
}

I wouldn’t expect this to happen since the user has provided their consent for us to receive their battletag.