I have the same issue, went to lv 13 this season but I'm playing in a lv12 league and my players had no loss of star rating. Seemingly it happens as you get to higher levels in the game and there aren't even players of your level (and around your skill level) to form a league.

It's a bit of a cop out game mechanic if you ask me and they should instead look to rebalance players across the servers so that leagues can be formed approriately.