you know, if its in the same spot, you have something behind that that is causing this or something is bleeding into this area. So, no where else its yellowing, just where it was before you removed all the grout and its back again.
Did you remove every particle of that old contaminated grout? If not, then its still there and moved over to the new.
Was that area wet, or I imagine you could not tell by only removing the grout.
I am really puzzled by this action. It has to be something behind or near or in that particular area.