I've played 500+ janna games on this acc, steady 60% win rate. And I'm currently 2 wins and 7 losses in placements :(
Edit: Just an update for anyone that cares/is having a similar experience. After the 10 placement matches I was placed in gold 3 (from plat 3 or 4). I lost another 3 or 4 games after that.... but then I started winning again and I'm getting 23-26 LP per win (6w 1l so far today).
Tl;dr - if you're placements are rough don't worry about it, if your situation is anything like mine your MMR will be fine - just matchmaking giving you some tough placement games.