The "x-factor" when breeding with Stella de Oro is the source material.
Stella sets pods easily, and in my experience, the seedlings tend to bear a strong resemblance to Stella. If the nursery stock is not deadheaded, these seedlings will grow within the clumps, and will likely be sold as Stella.
Stella has been massively distributed and tissue cultured, (often by indiscriminating sellers who have no particular interest in daylilies) making it difficult to determine whether or not the plants sold are the true cultivar. The name Stella De Oro has practically become a generic term for "yellow reblooming daylily" in the nursery trade, the horticultural equivalent of "Q-Tip" or "Kleenex".
My own clumps of Stella pre-date tissue culture, and the flowers appear to be true-to-type, but I would hesitate to pull a random fan and bet on it in a DNA test against a known, true sample.
By way of example, the database here contains several images which are highly suspect.
@sooby Where was that Griesbach article printed?
