Thinking like spinach or beans, would these foods lose nutritional value if I were to dehydrate them and grind/powder them to be included in stew?
No, Only water and heat reduce the nutrient content. There are other processes that will reduce nutrient content of food, but those are not used in cooking, to any degree.
Water leeches vitamins and minerals, if heated.
Heat deactivates naturally occurring enzymes beginning around 105 degrees.
Dehydration, especially at low temps(below 105 degrees) actually concentrates nutrients, and preserves enzyme activity.
So, to dehydrate, then powder, there will be no loss of nutrients, however, once added to a stew at greater than 105 degrees, you will begin to lose enzyme activity.
EDIT to add “and preserves enzyme activity.”