It seems TTS intonation is random: very often affirmative sentences sound like questions and questions - like affirmative sentences. Every time I have to check for a question mark, because intonation is wrong. Is it possible to fix it in the future?
No, it is not possible to fix. There is only one TTS I am aware of that gets Russian questions right most of the time. It is rather old (it is not sold anymore) and has other problems. Unless Duolingo decides to go for live recordings, the intonations will be like that.
Though, they can probably switch to a different TTS that makes more mistakes but read all sentences as statements.