Try another wash but don't add any detergent or booster. ..just clear water....do not dry it until you know its gone.

Did you already machine dry it after the wash?if it wasn't yellowed out of the package than it could be carbonate residue...carbonate residue and heat equals yellowed textiles.

I did dry it. I didn’t notice the yellowing until I saw it on the bed in the daylight.

I’m getting a new Electrolux FL next week so will run them through first thing.

Congratulations on the new ELux...

Yay, I’m guessing that means I chose a decent new set? It is so hard to convince my husband to make a decision, he is frequently overcome with analysis paralysis.