Registration foliage determination really applies to the garden of origin and doesn't necessarily translate to other climates. I believe you are right that the "middle" zones have more trouble with daylilies that try to grow during warm spells. It's too cold to have that problem here, no daylily tries to grow during winter, all are dormant. We can get down to the -30C (-22F) range with very little snow cover too so I think the lack of winter thawing is what helps.
