Just had this reply from Tilemaster
Thank you for your email.
I have known this happen a few times with grouts, not only Grout 3000 but products made by other manufacturers. Essentially all it is, is some loose laitance / contaminant left on the face of the grout that wasn't removed during the washing off phase. As the running water hits the face of the grout, the loose particles run off with the water. This will certainly stop once the contaminants on the surface have either been fully removed by the water, or equally if you were to rub along the joints with a slightly coarse pad, such as a washing up sponge, this will remove the very slight film left on the surface of the grout.
We shall see what we shall see!