Differential should have no effect on your problem. This is a mid wing straight (no dihedral) aircraft so it does not have a self righting tendency. As your bank increases the wing on the outer portion of the turn will be producing more lift and it will try to roll into the turn. You will notice this same tendency on all aircraft just at different bank angles. It should not be excessive but is noticeable. What you need to check though is that the controls center properly after deflection, and that the plane balanced laterally.