A good few of the gardeners in my area have had exactly what your photo shows, and the general consensus is that it's a soil deficiency of some sort; these are people who've grown on the same land for generations and so know all the foibles of the ground and weather hereabouts and it's certainly not neglect or drought. Most agree that it's down to leaching caused by the unprecedented rainfall over the past few years.
As Gooner says, top 'em and dig as needed; those I've been given to eat have been very good spuds indeed, so not a total loss