Yellowed spot on a white 100% cotton comforter

There was a stain on my comforter, origin unknown. I spot treated it with pure laundry bleach. It immediately turned to a dark mustard yellow. I continued to treat that section and very slowly the spot started to fade. I left it for about 4 hours. When I returned the spot was gone, but the area of the white cotton had changed to an overall very, very light yellow which is noticeable against the rest of the white cotton comforter. I rinsed out and washed that area, but the discoloration persists. Can anyone tell me how to get that discoloration out and to return my comforter back to it's original pure white "sheet" cotton look?
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