I have the old holders with solid backs and I haven't seen any similar effect. I suppose if it was there, it would be evenly spread all over and so, for practical purposes, invisible. I did get some slightly, very slightly under-fixed patches until I drilled a hole in the centre to help the flow. I can see where the ribs were on a wet negative, but this vanishes when they are dry. All I can think of is that you are getting slightly less efficient removal of the backing where contact occurs. Maybe your agitation is a little too gentle to penetrate this narrow gap. I swing the tank upside down so that the liquid falls down through the film assembly and then swing it back again to repeat the effect. Do you use a pre-soak? I don't see how this would make a difference, but it's another factor. I always pre-soak. Does Ian? Is your tank at the same temperature as the developer? Could a cold or warm holder be altering development slightly where it touches? A pretty desperate idea, I know. I'm afraid I've run out of ideas, however improbable. I do hope you find a cure.