"What do you roast?"
It did not accept Cosa arrotisce? - using the polite form, but how were we to know that they wanted the informal tu form?
Perhaps Duolingo should adopt a convention so that sentences with a first name such as "What do you roast Roberta?" would always have a "tu" form and those with a title such as "What do you roast Mr Smith?" would have a formal "Lei" form.
. The applies to the other languages too.
I don't know if it helps, but while the pronoun is not always required, including the pronoun when using the formal you 'Lei' can clarify that you are indicating 'you' and not 'he, she, it'. I cannot say whether that would've worked here or not though. I like your idea though because it would provide us with more practice for the formal you as well as clarifying the issue.
But the clue they give does not indicate that it is an irregular verb. The clue they give is arrosto, which is confusing as that is not how an irregular verb would be conjugated. The clue should be arrostisco, because that would be the correct form for an irregular verb. Instead they give arrosto, which is actually an adjective or a noun, but not a verb.