It's a bit of both. A friend of mine is involved with food sciences for Ministry of Agriculture and Food and he was the person who first told me about it as he thought I should mention the shelf-life in my books. He's a food geek and like a walking encyclopaedia of the molecular structure of the things we eat. Next time I am chatting with him I'll ask if can write the explanation down for me because it was somewhat technical (and admittedly some of it was way over my head).
That said, I decided to put his comments to the work in my own kitchen and see how things fared. While his comments were appreciated, they lost me a little, and I wanted to see if this flavor loss would be noticeable to the average person and if it was enough to need a little note about it in the book. I'm all for being able to make my foods 5-6 months ahead of time so it was important to me. I take my work very seriously.
So, what I did was hold back some of the potatoes from three batches over an eight-month period. The potatoes were roasted without seasoning other than salt and pepper. Then Bryan and I taste-tested them. Bryan's was a blind test but I knew which was which. I have to say that I was a bit surprised when he did notice a taste difference. The potatoes that had been dried/stored the longest had less flavor that those recently dehydrated. Now keep in mind that the amount of flavor loss also depends on the time frame. All three test batches were stored in the freezer. There was also a color variation greater than what was present at the first time of storage.
Exposure to air, light, how they are dehydrated, how they are cooked and such are all factors in how long before you have significant flavor loss. From what I understand with shelf-stable canned product that is freeze-dried will have a longer shelf-life until that can is opened. I haven't put the commercial freeze-dried product to the test but I have experimented with the commercial dehydrated and had similar results to home-dried - freeze-dried is a bit of a different beast.