Position changed after a re-line in rank

First, the new UI takes me a long time to get used to. Recently, I found that when I re-line for a game, my pref position may change. For example, I choose mid and jungle for a game. And someone fail to select a cham and the system auto re-line me for a new game. The new game I become support, and it not just happened once or twice. And I am not sure it happends to other people. I hope this problem can be solved,