Thanks Brian. Got it. I haven't heard of this with HPR Baryta or IGFS. Is that because I haven't looked?
Why it happens, is up for discussion, and has been discussed on these forums in the past. But, Hahnemühle Photo Rag Baryta doesn't appear to have any OBA content, and while Ilford Gold Fibre Silk appears to have more than it used to, it still has a lot less than Epson Traditional/Exhibition Fibre. It could also be where the OBA dyes are in the papers (throughout, top, base, surface coating), when they are added, the type of dyes they used, who knows?
If you do not have a spectrophotometer and software necessary to measure paper spectral reflectance, do yourself a favour and download Ernst Dinkla's Spectrum Viz app. It has measurements for several hundred papers. (That peak around 440 nm is indicative of OBA content.)
http://www.pigment-print.com/spectralplots/spectrumviz_1.htmIt is a very useful tool, and it is very good of Ernst to put all this data out for anyone to use.
Brian A