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I had this exact thing happen in Duolingo. I remember being annoyed, because in school I had learned to say "comment t'appelles-tu" but Duolingo taught me "comment tu t'appelles." Then it seemingly randomly switched back to the original one I knew, like I should have been using that the whole time.

What happened I think, is we left the "s" off of the end, so Duolingo marked it as wrong, but rather than add the "s", it gave a different correction (the more proper way to say it). If you had put the "s" on the end, it would have been accepted as correct.