Some items turning yellow in my washing machine
In most cases items that turn yellow or have yellow stains are acrylic. This is not a problem that has been caused by your washing machine, but is actually caused by prolonged exposure to sunlight.
Unfortunately, once these marks appear, there is nothing that can bring them back to the correct colour, your only option would be to dye the item.
The only preventative action you can take is to avoid exposing any acylic item to sunlight.