It's not mistakes. Duolingo is a crowdsourced site.
It means that the course contributors, that are users like you, try to include all the possible answers, but it's impossible to be an human being and to think about all the possibles answers for a sentence, so they added a system where you can report the missing alternative answers. And they add them after a while. So, it's not a mistake, they add it after your report.
I'm French, "tu" is not always so intimate, as I use it all the time to talk to strangers on the internet, young people also use only "tu" when adressing to another young. But it's not really the topic here.
Just report when they forget "he" or "she" for "dia", and it'll get fixed.