The question would be, are the borders also yellowing? If they are, it isn't toning, which affects only image areas. (There are some dyes that are marketed as toners but this isn't one of them.) If white unexposed borders are yellowing, something else is going on. If the print image areas just look too yellow but the borders are not (compare to an untoned print) then it could be what I ran into. I'm not sure why sulfide toning makes inadequate fixing more obvious or fresh fixer more critical (my fixer use was well within the listed capacity) but it sure seems to do so in my experience.