"A mountain in winter is a dangerous place."
It's still not accepted now. I have reported it too. Duo also doesn't accept ”冬” and ”危ない”.
Because the subject is the winter mountain, the は comes after the mountain. You wouldn't be able to have two はs in a sentence like this. There is only one subject.
We cannot use に because it shows direction or action. i.e. ふゆに山へ行く。I go to the mountain during winter. The example sentence is a statement and isn't showing action.
The の shows ownership. The winter's mountain. That way they can be connected.
There are other ways to mix up the sentence to say the same thing while using different particles. But this sentence needs the の .
We often say things like this:
夏(なつ)の海(うみ)はきれい。 The ocean during summer is pretty.
秋(あき)の紅葉(もみじ)は美しい(うつく)。 The autumn leaves are beautiful.