Ranked series gold 1 into plat 2-2

Hi there, I was in gold 1 series and I was 2-2 in the games, I then went to play my 5th and final game in my series won it and said I had been advanced for my series, for gold 1 again? Has anyone had this problem? I'm pretty sure its a bug? Don't really want to play another series for plat again if its going to do the same again? Any ideas?