The problem with a lot of coated racks is that the coating erodes or cracks, often so subtly that you can't see it... but it exposes the soap to metal. I had racks like that and it was some of the only DOS I ever experienced.
I use coated racks too, but they do erode. I actually think the soap itself is one reason for the erosion of the coating. I always put a layer of plastic mesh between the rack & the soap, which lifts the soap up off the rack just a tad. In spite of that, I have seen coating on a rack erode in as little as one year from using it for soap curing. So I am glad that I learned to add the extra layer between.