Matchmaking.

What is up with matchmaking, I went into my promos to plat 1 gaining 26 lp per win. I get into promos, and am placed with a midlaner who has a 16% winrate over 20 games played, and had not played ranked any previous seasons, and is also only level 46. How is it possible to be plat 4 with a 16% winrate? Are you not able to demote entire tiers anymore? How is starting mmr calculated?? Why does this only happen in promos?