I was about to search for this thread as well. Glad you found it.
I have a static team of 4 that we play and noticed this. Some of it is due to debt, the tank tends to not die as often as the defender, so the defender gets a little more debt, etc... What we started to do was give the star the person that levels first. They keep it until someone else levels before them. It isn't a perfect system, but it seems to keep everyone pretty close.