So, I was playing 3v3, and first turn me (Greece) and Germany both attacked Austria, I won and it showed in battle report and then I saw this

Next turn we both attacked empty Austria again and same thing happened, I won and my units didn't capture Austria, instead they went in Slovakia which I didn't attack

So finally third turn I attacked alone and took it

So my conclusion is that this is a bug which can happen when 2 opponents attack same country, but I'm not really sure how or why this happens.