Implicit solver for gamma volume fraction equation gives alpha1 > 1.
 

I am using the Implicit solver for gamma volume fraction equation, MULES::implicitSolve, with a simple 2D problem where I have a source for alpha1 that changes with the temperature. In this problem it is convenient that alpha1 should always be lower than 1. I have tried different numerical parameters and schemes, but apparently this solver does not fulfill the conditions that alpha1 should always be <=1. Is this a bug or just a limitation of the solver?