I have seen yellowing after toning and washing with Ilford MGWT FB and direct sulfide toning. It was definitely yellowing, not toning, as the white borders were very obviously affected. What was puzzling is that sometimes it did, sometimes it didn't. Turned out that the process seems very sensitive to fixer exhaustion. Fixer that tested fine with hypo check but had been used a bit, but not overused according to my past experience, would result in this yellowing. Fresh fixer did not.

The good news is that re-fixing and washing seems to remove the yellow, but doesn't affect the toning.

I now limit fixer use to no more than 15 sheets of 8x10 per quart of film strength (no hardener) Rapid Fixer. I began to see this yellowing at about 20 8x10s.