Not sure Alberto, as I only use enamels normally. However, all metallic paints can suffer from partial separation of the metallizers in the pigment when thinned, or if a clear coat is added over the top, which is not really noticeable in general 1:1 use, but looks more apparent in smaller scales.
For a 'painted silver' finish, I normally add just a touch of white, which takes away the metal shine, but also prevents the break-up of the metal pigments. You could try this, but test it on some scrap first, and adjust the mix as required.
But, it might be that, as your paint is 'Aqueous' acrylic, thinning with water might be better - the thinner used might be causing the separation.