Now that's interesting - I've had the same thing happen in the past
with Epson inks on Olmec gloss paper, the inks gradually spread into
I assumed it was a bad paper batch (liked the paper, but didn't buy
any more), wonder what causes it?
Dye migration is often related to elevated humidity conditions, but some papers incorporate moisture attracting humectants to promote rapid ink absorption during the initial printing step. The image receiving layer is then chronically prone to dye migration even at moderate humidity levels. Other paper formulations attempt to contain the dye migration problem better by incorporating additional mordanting technology (chemistry that bonds with the dyes to help anchor them in place). Pigments are large enough particles that high moisture content and residual solvents in the paper generally can't dislodge them. The "bleeding" problem you have observed is essentially a problem only with dye-based systems. Hybrid pigment-dye systems also exist (often marketed without disclosure that they aren't fully pigmented inks), and their humidity related performance tends to land somewhere in between.
I wrote a technical paper that has some test methdology information about this phenomenon. You can find it on the Wilhelm Imaging Research website. Here's the direct link: