Inkjet papers contain dye mordants much like dye-transfer paper, which is what I transferred a q&d Ultrastable test to. Like your experience, a brown stain appeared in the next couple hours.
I have seen these compounds (diazo/azido) referred to as dyes before.
If your fixed-out FB paper was treated in a hardening fixer containing chrome alum or the like, you could expect this to have a mordanting effect as well, though significantly lower. It therefore might be interesting to observe what happens with photo-paper that was fixed out in a non-hardening fixer.