Lane Position Trading is Broken!

So i was in a match of ranked and my ADC was afk, but he was a duo with my Jungle. (he said he would be back so do not remake) we didnt. so around 3 min he returns, during the 3 mins I farmed the lane. When he returned I let him begin farming. but at the post game lobby the match placed me as a position trade and put me as a lane swap for ADC. making me lose LP for a role I didn't play. Why?