American Swiss sandton defective ring

Hi, my fiance bought me a moissanite engagement ring with 9ct white gold on the 10th Dec 2021 from American Swiss Sandton . 2 weeks later the ring started turning gold (I do not wash with it, or even shower with it). I then took it back to the store in Feb for them to advise , they then said this should happen as it is white gold but my questioned remained, why would it change colour when I have only had the ring for 2 weeks?. They then said that they will take it for re-plating after I insisted that something must be wrong with the ring .I suggested that they do a double replating. They assured me that after the replating it should be fine. 2 months after the replating , it started turning gold again, only this time even worse as it wasnt only the band but the hooks that hold the stone too which turned gold. We took it back to the store and they said they would need to take it to CPT for quality assessment, they then gave me a call on 13 June saying that they see no defects with the ring and it must be wear and tear. Wear & Tear after 2 weeks? and again after 2 months AFTER double re-plating ?? We suggested that we exchange the ring or get a refund. However the store manager Yulan said that this is something they are unable to do and that his area manager would not recommend this but would suggest that I choose a completely new ring because we might have the same issue again. Our question is, why would an exchange give us the same issue if there isnt a problem with the rings made?? They are still insisting that we choose a entirely new ring, and even said we can go with a different stone etc. Since when do stores choose for their customers? I love the ring my fiance gave me and I should be allowed to get an exchange of the same ring. If they are worried that the same thing will happen then our argument that the white gold moissanite rings have a defect stands and in this case we should be allowed to get a refund. After such a frustrating experience, we asked for a refund however they refuse because they can only give refunds on defective rings. Please can I get assistance my ring is still within 6 months warranty, this is supposed to be a seamless experience for us as a newly engaged couple but American Swiss is making it a nightmare.