I've noticed this, too. But my thoughts on it are that while I'm sure there is some in-breeding I suspect hybridization is contributing to this look. When I first raised RCS, none of them had the pale stripe down their backs. And the shrimp I saw from other hobbyists did not either.
By 2003-2005, FW dwarf shrimp keeping was taking off among hobbyists, and the shrimp were commanding prices that got them noticed. Dwarf freshwater shrimp began to come in from many sources. The first time I saw these dorsal stripes very prominently was on "Rainbow Shrimp". My sense is that this was a sort of catch-all name for a variety of colorful Neocardinia coming in from different locations. Since Red Cherry Shrimp were deemed highly desirable, either distributors or sellers may have segregated the red ones, and they got mixed in with the true red cherry shrimp, at any stage along the distribution path, including in hobbyist's tanks. Personally, I began to see more solid red coloration at the same time the cream colored dorsal stripe became prevalent. Some of the "Rainbow Shrimp" were far darker, more of a brick or mahogany red. Also, the color of the "early" RCS seemed to be a series of spots or blotches, which were thinly scattered on the males, leaving quite a bit of clear area, and more dense on the females, giving them a more intense color. As females aged and bore more batches of eggs, these areas would get more dense and her color would intensify. Some of the "Rainbow Shrimp" had very opaque coloration, with every segment of their exoskeleton being colored. I wonder if that is part of the more solid colors seen these days. I've seen some shrimp labeled as Red Cherry Shrimp on the biggest auction website, and to me, they don't even look like RCS, because they're a murky, almost aubergine color.
Yes, some of the intensification of the color is from selective breeding, but I'm kind of skeptical that the breed has stayed "true".
Eh, just my 2 cents.