You're not growing in the coldest climate by any means, but it sounds as if it might be one of the hardest on daylilies. Regarding the hybridizers you cite, I can't offer any specific information—I don't grow any Mitchell or Carpenter daylilies, and I only recently acquired my first two Norris plants, Belle of Ashwood and Remembered Kisses last fall.
I don't think what you're seeing is due to natural genetics, per-se, but it's not happenstance either. It's the genetics that hybridizers have created, having more to do with artificial culture, the selection process, and human nature.
For a plant species to survive in nature it must not only withstand cold and heat, but must also be attractive to pollinators, fertile, and able to recover from stress or damage of all kinds, both physical and environmental. As gardeners, we try to minimize the hardship that our expensive acquisitions must endure.
My climate is not conducive to the best bloom quality, but it's probably one of the most comfortable places a daylily could wish to be grown, so I have had very few unexplained losses, and those were mostly associated with plants which "stalled" during shipping and couldn't seem to recover. This itself may be indicative of a "sensitive" daylily. I treated quite a few diploids with colchicine in the late 80's, and have had multiple fans of a few cultivars die in the process. When I talked to experienced hybridizers, it turned out that the same cultivars were behaving similarly for other people as well.
This characteristic I call "touchiness", or "failure to thrive" would seem to be inevitable when seeds and seedlings are coddled in near-antiseptic indoor growing conditions and anointed with fungicides in order to ensure that the maximum number of seedlings survive. It might also be a result of highly competitive race-to-market breeding programs which may tend to encourage breeding with young, unproven parents, or substandard plants with attractive/unusual flowers. Desirable plant habit and toughness can be bred back into a line, but it requires hard choices, and not everyone seems to be placing a lot of emphasis on it.
Whenever suggestions of a new, exciting floral trait are discovered, other aspects of the plant seems to be de-emphasized, and buyers eagerly snap up untested, second-rate plants with faults such as low bud count, weak scapes, ratty foliage, misshapen, poorly-opening blooms and muddy colors, just to get that one, new and desirable trait.
The impression I get is that before daylily hybridizing became America's favorite pastime, new cultivars were field-grown under fairly harsh conditions, and evaluated for a longer period of time before being brought to market. Greenhouse culture was unheard of, and seedlings took at least two full seasons to bloom, sometimes three. Hybridizing was the province of a few established nurserymen, those with a scientific bent, and amateur breeders who were mostly interested in having a fascinating, engaging hobby which might help pay for itself. Things have a tendency to change when the annual catalog represents a livelihood.
Ever since I can remember, people have been preaching that more emphasis be placed on plant habit and overall health and vitality, but how many breeders actually have the willpower to resist the siren song of the new and fantastic?