A few months ago I mixed a stock solution of 200ml rehalogenating bleach using a ratio of 7.5g/l each of Potassium ferricyanide & Potassium bromide. I used deionized water for this.
It worked exactly as desired at the time, using a dilution of 1+10.

It was stored in a dark amber glass bottle at room temperature. I dug it out today and was surprised to find it a dark chocolate colour. Testing it on a scrap negative, it didn't appear to bleach the image at all, even at full strength.

I'd been under the impression that the stock mix would keep well - not indefinitely, but for a year or two.

One faint possibility, I suppose, is that I stored the working strength solution and disposed of the stock last time I used it, although the bottle I opened was clearly marked as stock.

Any comments? Have I simply misunderstood the keeping quality of these bleaches?