Yellow marks appearing on a white synthetic garment, usually acrylic caused by exposure to sunlight and not staining as such.
This problem is not caused by your washing machine and no service or repair will resolve this problem, only care of the garments will prevent the issue.
Damage is irreversible
Acrylic fabrics when exposed to prolonged light or heat sources can turn yellow, due to damage of the fibres.
Do not expose synthetic garments to prolonged sunlight or heat.