This has been causing me confusion this season. My players always have the same star rating, despite whatever competition I'm in. What I mean is, its the same for my league matches (level 13), and my champs league matches (level 12).
I'm playing in two different levels with the same star ratings.
I reckon perhaps, that if that scenario happened, then I'd probably just lose the usual 1 star as according to my champs league level. Going from level 12 to 13 via champs league.