Epson pigment inks (Ultrachrome/HDR) are not designed for long hours of display in direct sunlight or outdoors. You must use solvent based inks for those uses. I'm not sure what is being out gassed in your case, but the few times I have had to tear down a framed print due to fogging of the glass I don't recall a yellow residue. Normally I would suggest giving the print a full two weeks to out gas but if the print is going to be in direct sunlight like you describe I really don't have much to suggest other than to move the prints so that they aren't in the sunlight.
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