For several years I have been growing a Buddleja davidii alba shrub in my
garden which each year has produced plenty of white scented blooms, all
white and only white.

To my great surprise this month I observed that some branches are carrying
white blooms while other adjacent joining branches are carrying light mauve
blooms.

What has happened to this shrub ?
Is it possible to get it back to carrying all white blooms in future years
simply by pruning off this year's branches which are carrying the mauve
blooms ?

Anthony ( 30 km east of London )