Mark, from Aardenberg Imaging is probably the one to say more definitively, but it certainly sounds like gas induced yellowing of OBAs. I'd be rather surprised if they had changed the paper brightness overall without others taking notice, and batch to batch irregularity of the order it sounds like you are describing sounds unlikely too. Plus, it is consistent with the age of the prints, in your observation. As Mark has posted here, paper brightness is not really on the radar of Wilhelm's testing regimen, as they are looking almost exclusively at ink fading results. I'm sorry to hear about this, but it certainly seems like, now that ink fading has become much less of an issue, that this is one of the more significant print permanence issues to be wary of.