"Leo, haluatko lisää täytekakkua?" should be accepted as correct, got penalized for skipping "sinä"
Yup, did you report it using the flag? That's how the course is improved.
Yup reported. I usually write a comment here too, so they know what the problem is.
(I keep asking users who write here that their answers weren't accepted, since lots of people haven't realized that's how it works. If you write "reported", I know not to go on about it :-p .)
I wrote "Leo, haluatteko te lisää täytekakkua". Duo said: "Wrong". My GF who is a naitive Finn said: "Duo is wrong"
Sure, I used formal you rather than informal. Still though, formal should count.
In Finnish, we never use the formal te with a person's first name (unlike in German). It is pretty uncommon anyway to use te (unlike in German). And if you're on first name basis with someone, you're way past the stage where you switch to sinä.