The nightshades contain calcitriol (1,25-dihydroxyvitamin D, not cholecalciferol aka vitamin D3), which if I recall correctly, is 1000x more potent than D3. Production of calcitriol is extremely tightly regulated by the kidney, because too much quickly leads to calcinosis. See the linked studies in the first article, where they specifically talk about calcitriol causing calcinosis in animals. Look to the human drug information on places like Rxlist.com or Drugs.com to see what excessive calcitriol does in humans.
We are not supposed to bypass the body's control mechanism for calcitriol. Nightshades do this to our detriment.
Calcitriol will not show up on a normal blood D3 test, and the transient elevations of calcitriol from eating nightshades are something that doctors wouldn't ever be looking for.
I'll write more on this later, I'm actually ironing all this out for my WAP presentation in less than a week.