I've wondered about this for many years, and you're just the person that can answer this...I hope.
Why do all starches thin out, after thickening a liquid? They all vary, with potato starch being the worst, in my experience, thinning within minutes, while still hot, while wheat starch (not flour) seems to stay thick longer, and even re-heat some, though it will still be thinner. Arrowroot also thins out, and I've seen conflicting statements in CBs, one saying it reheats well, another saying it will have to be added again if reheated. Tapioca starch, which I use the most, is in between, as far as keeping its thickness. I made a soup in a friend's kitchen recently than brought this to mind again - CI's mushroom/wild rice soup, with corn starch (which I never use because of this), which had thinned out considerably by the time it had cooled, and after chilling overnight, it looked like it was made without thickener at all.
Is there a chemical(s) in mushrooms, meats, or other foods, that causes this thinning out? And why doesn't it do it with flour, which gets thicker as it cools, and you almost always have to add some liquid when reheating, or it is too thick! Is it more the non-starch compounds in flour that thicken things, and keep it that way? I realize that the actual starches from one plant to another vary, thus they behave differently, but they all do thin out, at least some, and it seems that mushrooms thin them out more than any other ingredient, in my experience (or maybe I just make more mushroom soup than the others).