Maybe you've been unlucky?well you've been pretty lucky is all I can say - I've been trialling his bees for three or four years now and varying degrees of chalkiness is apparent. got two genetically separate ones here now and I'm clearing literally piles of chalkbrood mummies out.
Not had a problem with chalk brood with any of his queens, my mate can say the same and we've 'trialled' one or two . I've a four year old still going strong, they are beautiful queens.