Why is MMR of 1 position effecting placements in others?

wasnt this the whole point of positional ranking? so that you can place each individually? with each individual role having separate mmr? yet i place my jungle 1st, get p2, lose some games, get demoted, say screw this im going back to top lane. I start my placements for top lane, only get 15 lp per win unlike the 70 per win i was getting flr jungle. if i lose a game it takes lp away from each position except top while in placements and even subtracts from roles i havent placed yet. this whole system is broken. s9? more like preseason v2.