[SOLVED] Mass below a multimap

I have a scene in which I have two sets of independent dofs (x and d) and one set of mapped dofs (p). The mapping is a multimap with the following expresion: p = Bx + d
I also have a projective constraint on d.
I have tested all of my components and everything seems to be working fine, but when I simulate a basic triangular mesh under gravity with a UniformMass on p the triangular mesh gets a strange aspect after a few steps:

I am using a CG solver. If I understand correctly how this works, the gravity force is mapped as a normal force, but what happens with the mass matrix during the solver? Is a UniformMatrix supported under a mapping?

I can provide more details if needed

Thanks