I'm dealing with the same thing. I also keep going through cycles of the ascorbic acid treatment. But a couple weeks later, the stains return. I'm probably not adding enough sequestrant, but I'm sick of dropping $20 on a bottle every week.
Heres what what happened this last weekend though. I got so sick of dumping in more sequestrant, I stopped. And this weekend, not only did I have the staining, but the water turned yellow as well. So I stuffed the skimmer with paper towels, and within a few hours, the paper towels turned nasty orange / yellow. So I thought I might be onto something. So I ran the pump continuously 24 hours a day and kept switching out paper towels. After about 3 days of this, my water is back to blue / clear (and the paper towels are no longer turning yellow. I still have staining, but I'm hoping I caught the majority of the metals in the water by doing this process. I'm going to do one more ascorbic acid / sequestrant treatment, and I'm hoping it holds much better / longer this time.
In short, I'm wondering if the way to approach metals in the pool is by driving up ph, dumping a bunch of chlorine in so that the water turns as yellow / orange as possible. Then catch those metals with paper towels in the skimmer. Then ascorbic acid to clear any stains off the pool walls.
may have to repeat this a couple times, but hopefully it's a more effective and cheaper long term solution.