Basically, Phenidone is less stable than its chemical cousins, and that is stated in Haist. It goes bad in the concentrated form more rapidly.
It also oxidizes more rapidly in alkali if O2 is present. The same protective groups that help the others in alkali, help them in oxygen.
If you want a good concentrate with long shelf life, use Dimezone or Dimezone-S. If you want it to last long in the tray, same advice.
PS. For those who have not guessed, I've done that experiment!. I can also add that Phenidone lasts a long time dry in coatings! It can be used as an incorporated developer. Done that too!