Here is what hints in that direction:
- Oxidized products of ascorbate have no distinct color, i.e. you don't see when it happens in water.
- We know that Phenidone/Dimezone S catalyzes oxidation of ascorbate, that's why it is superadditive
- We know that this happens more likely in caustic environment, hence you see it faster with metaborate
A very quick'n'dirty test would be, of course, if you would try developing a test clip of film with the soup once it precipitates.