The use of ascorbic acid in the food industry is wide spread. Why, because it readily scavenges oxtgen and therefore protects other chemicals. Because of this it is unstable in solution so making up a concentrate solution is really of no help. As has been stated over and over again on APUG the main problem is Fenton oxidation which is catalysed by iron and copper in the chemicals themselves and in the water used to make up solutions. Use only de-ionized water for making up solutions. Do not use distilled water as this is often made using copper equipment. The stabilitry of ascorbates is improved with the use of certain chelating agents such as salicylic acid and TEA.
My suggestions would be either to mix up the developer without the ascorbate and then weigh it just before use
use the two chelating agents mentioned and use only de-ionized water.