I got the same problem with my front hub too, recently got sent a pair from mack himself.
I did some research on why people have this problem with the front hub.
When building the wheel, you dont really notice anything but as the wheel is tensioned tighter, the hub shell stretches so slightly as the spokes are pulled.
the bearing are then not fit as tight into the hub shell as before.
I think this is because of the shell it self. its very thin and lightweight.
Even though it does stretch, you can still get a stiff fast wheel-set built up.
I spoke to @ekudduke about this too as he had the same issue and he told me to add a couple drops of locktite to the bearing, making sure not to make a mess of course. This has fixed the problem for him and myself.
Hopefully this is useful to other people with the same problem