Perhaps it's just a genetic varient in the variety. I would think the best indicator is what's happening with the all-important root.

It's a golden beet, and on one plant there is almost a full-sized root. So productivity isn't impacted. A little odd that it might be a genetic variety, as the two affected plants are right next to each other. If they were funny seeds, they could have ended up anywhere in the bed. I should add that those plants are now noticeable stunted. Doesn't seem to be spreading, though.