I might get hate for pointing this out again lol
Nadech and Yaya were obviously just friends for a long time until it recently turned romantic i feel so although there were rumors about dating other people, it would make sense that there would be no cheating rumors plus I feel their fanbase on a whole different level of devotion to shipping that pair. Plus their career was built off them being a pair tbh, they would not dare do anything to ruin that.
Mark and Kimmy although free of cheating rumors-- aren't free of third party rumors either. When they first got together, it was highly scrutinized that Mark was a third hand in Jett and Kimmy's relationship bc there was some flirting going on but kimmy excused it as just being friends. However, seeing how quickly they confirmed being together afterwards and how it took Jett so long to work with her again, I think it's safe to say there wouldn't be smoke without any fire. Mark also had rumors of cheating on or two timing when he was rumored to be with Vill too. They are a cute couple though, and the fact that they share such a more personal look into their relationship, they seem more like your traditional long term couple who is about to get married any day now.