No, it doesn't ring true. The commercial lab I mentioned above uses "a short stop, two counterflo rapid fix solutions, a hypo eliminator, three separate washes, and a photo flo final bath" to quote from its service guide. I have never had any indication that negatives developed by that lab, either in Xtol or other developers used previously, were even slightly underfixed.(posted 8951 days ago)I'm now at home and able to reference earlier correspondence with Dick Dickerson and Silvia Zawadzki, the former Kodak employees who wrote "The Genesis of Xtol" in the September/October 1999 issue of Photo Techniques. Their response to my inquiry stated: "With both TMX and 100 Delta, at high exposures, you are punching light into the bottom portion of the film (closer to the film base). Both of these products have very fine grain emulsion in this area to capture heavy exposure (not nearly as fine with 400 speed films where you don't see such a problem). With Xtol, these grains that are very small to begin with are only partially developed (because it is a fine grain developer). Result would be extremely small developed silver particles - and when such silver particles start getting really small, they do take on a distinctly yellow cast which blocks blue light."
Perhaps this problem hadn't been seen before Xtol because there was no reason to use a fine-grain developer with these two already very fine grained films?