The only way to avoid this is to not promote but I guess this is not something that you look for.

What you experience is normal because the stars reflects your team quality for that level.

Example: If you are level 5 and your team quality is 100%, someone who is level 6 sees your team as 80%. You went 1 level higher so you see your team weaker, however if you check the true quality of the players (the number after the percentage), you will notice that stays the same.