rank system

Hi, question here. I have a rank game which I get autofilled to top as my prefer position is jg. And I have both my jg and top in promo, I asked my top laner if he wanna trade so i can finish my jg promo and he did. Then I finished that rank game as a jgler and we won. BUT neither my jg or top got promoted, is this a bug or designed this way. It's frustring when you win a hard game and nothing comes back to you.