If you watch twitch streams, there have been many times were the streamer loses a match and they get the "promotion" notification at the end (wouldn't this donate rise in ladder?).
It happens only when you are playing arranged team, and it was always like that. Whenever you finish a premade team match, it displays your current team rank at the end, no matter if it was a loss or win. The "promotion" in this case can be safely ignored (bug)