While Xtol keeps for a long time in full bottles, it oxidizes in part full bottles. When a bottle is only 10% full the ratio of the volume of air to the volume of developer is over 80 times that when the bottle is 90% full, so the oxidation can go ever faster as the level in the bottle falls.
I ran some tests on ingredients of Xtol. 2 lots of 12g/L sodium ascorbate without and with addition of 20 g/L sodium sulfite were exposed to the air and the color change noted (see attachments).
After 37 days the ascorbate alone had oxidized to an orange color, possibly dehydroascorbate as dehydroascorbic acid is reported to be orange. The ascorbate plus sulfite solution showed much less color change.At 130 days the colors had darkened.I cold not get rid of the orange color by adding sodium sulphite.
I am inclined to favour this explanation for these color changes:
In presence of sodium sulfite the ascorbate does not oxidize to the orange compound, probably dehydroascorbate, but to a relatively colorless substance which may be an ascorbate-sulfite compound.
So it is the sodium sulfite which in some cases leads to the oxidative failure of Xtol with minimal color change.
Thanks for comment.