Position Trade in Ranked Game

As we all know there are five position rank introduced in S9.

So the issue comes: For example, I am a master tier player. I win 7-1 on my adc main so I got to d3 0lp after placements.

Now watch what I do,

1. I feed on my min position for 8 games, so I went back to Platinum 1 after 0 win placements.
2. I found a friend who plays mid well.
3. We Duo, I select mid as my primary and he select adc as his primary position.
4. I choose ADC in champion select and my friend choose mid lane (Do a lane swap)
5. We win the game, because I as a Master Tier ADC I can just ran over my oppoent, and my friend too, he ran over mid lane.
6. I got a lot ADC position LP, because the system thinks I main mid but play ADC instead.
7. My friend got a lot mid position LP, same reason.
8. Even if you lose that game, I will lose less LP than I queue with adc as prefer role.

Keep doing this, until I reach d3 100lp on my adc position, play one more game, and then I will get to promos.

GG

Riot, Please fix this