The missing yellow has returned. Just in time for the last few stitches of the toe graft.
So I now have a "pair" of socks that I deliberately made not to match, but that the knitting goddess has decided will really not match.
I unwound the remaining yarn, & all the yellow was present & correct. (2 pattern repeats worth) So if I had started the second sock without re-winding the ball, I would have had one sock with yellow where there should have been yellow, & one sock half and half. I think if I had to choose, I would rather have what I wound up getting. But still, a fraternal pair would have been nicer.