Even Google Maps indicates that Jomtien starts right next to Phra Tamnak: https://www.google.com/maps/search/jomtien+demarcation+map/@12.8978136,100.8519429,14z
Where you are correct is in the actual name of the beach. But then you wrote "Jomtien starts".... making yourself wrong again 😊
Then you mentioned Phra Tamnak beach....wrong again. It's YinYom beach.