Phenidone is a chemical called a hydrazone. Hydrazones in general will hydrolyze in alkali back into their parts. Since phenidone is a ring compound it merely ring opens and becomes inactive. Dimezone and Dimezone-S have blocking groups at the site of hydrolysis and are therefore far more stable in alkali than the parent phenidone.
I know, I know, you all hate organic chemistry. Go ahead and use phenidone developers. They will last - a while, and then organic chemistry will bite you on the butt just when you least expect it.