You may already know this, but when you use ascorbic acid, whether it's pool store type or from a chemist shop, it is suggested you drop chlorine levels to zero (it eats chlorine anyway). Then you bring the chlorine up very veeeeeery slowly to prevent the stain from re-forming -- and expect that the process of the AA will CONTINUE to eat your chlorine at a higher rate than you expect. Eg. if you just go right back to full chlorine in a few days, or if you don't have a FULL dose of Jack's Pink/Magenta whatever (meaning 2 bottles, not one, if your pool is in the neighborhood of 20,000 gallons), in about a week or two, you might see the stain re-form, IME. If your PH raises quickly, you may also see stain reform.
In other words, if it is actually a metal stain, eg. iron, you will need to come back from stain treatment very slowly, can't shock for at least 2 weeks, will always want to keep your ph on the lower end and will need to absolutely MAINTAIN an effective PPM of sequestrate (eg. Jack's) via weekly additions of sequestrate. It kind of sucks
Now, I have well water, so I'm stuck with this for the moment. In your shoes, if you know no fill has ever been metal contaminated, I might try a new ladder (not that expensive) and also check your light for corrosion. Did you at any point use pucks? Sometimes they park and corrode things.
If you'd like to hear other's experiences about the way to remove stains through a full AA treatment, use the search bar -- there's lots of threads. That might help. However, you'd usually want your own drop kit to be testing while you're working on these kinds of issues.
The pool store's metal detection sampler also SAYS MY POOL has no metal but guess what - it's WRONG -- my fill has .5 ppm iron -- in the pool, it is just sequestered, so doesn't read, but precipitates if the ph drifts too high.
Aquarium stores, btw, sell iron test kits. But a FAS/DPD kit for the rest of your chem would be a better first investment, IMHO. Good luck!