if the adjective is "alone" after być/jest ... adjective takes nominative.

It can be: "Dlaczego jesteś smutna/smutny (when you talk to a woman or to a man respectively)?"

Well, I believe that "smutny" must have been there, then.

See this topic and if you have the chance to add a screenshot, that would be really helpful. https://forum.duolingo.com/comment/27952965

There are more equally correct answers here, the algorithm doesn't necessarily have to put "smutna" there. It can be "smutny", for example.

Although a moment ago this used to have the Formal You answers as the starred ones as well, so that may have been confusing, true. Fixed.

Currently the starred answers are:

Dlaczego jesteś [smutna/smutny]? (singular, female/male)

Dlaczego jesteście [smutne/smutni]? (plural, no men/at least one man)

One of them has to be possible to be created.

It's perfectly synonymous. However, some natives can consider it a worse option, either treating it as a bit colloquial, or saying that it's a Russicism. Most wouldn't bat an eye, though.