It sounds like two things have changed: Your local water, and the formulas used by the manufacturers of various hair care products (both the ones you use at your salon, and the ones your customers use at home).
Given the very small amounts of chloramine they’re using in the water, I’d suspect the hair change is due to the manufacturers reformulating their products. If it’s due to chloramine, then hair stylists in the large regions of the country that have been using chloramine for years would have likely noticed this odd phenomenon, no?
My assumption is based on how perm’s work(they use or used to use, amonia…the waving solution breaks down the cystine linkage and then the neutralizer oxidizes or rejoins the linkage in a new form(curl based on the rod size). My guess is that since ammonia is poured on the head daily or several times a week, it also breaks the cystine linkage, and as the client lays their head back against a chair or on a pillow, bending the hair into a new curl formation and oxidizes by drying, or just bends overnight even when already dry, a new curl exists that was not there before. Most people don’t lay on top of their heads, so no bending occurs, thus no new curl on the top of the head.