It may depend on the composition of the complete formula, concentrations, how much preservative etc but on its own yes Phenidone is apparently unstable. POTA is just Phenidone and Sodium Sulfite and is quite short lived.
Probably because POTA has no hydroquinone which is regenerative of either metol or phenidone.
I think Ian Grant, below, explains it all well.
Regardless, as to the situation of the OP, FREEZE IT!