Why do usernames of accounts you visit sometimes include your username?

I recently suggested Mastodon to a friend and she is using it on IOS. She sent me a picture of her visiting an account and the name shows as @username@herusername@mastodon.social

This doesn't happen for all accounts and I personally never noticed it. Is this a bug or is it supposed to be that way ?

Edit: She is using the "official" Mastodon app