Yes in phuket it IS mandatory to wear a mask when on a motorbike and you can be fined and even blacklisted if caught without one. The phuket governor issued the mandate and rules well over a year ago now and as far as I know it is still in effect. I have seen thais who work in covid hospitals/ vaccination sites take photos of foreigners without masks and their motorbikes at stop lights many times while also giving them a lecturing!
If op continually refused to wear one, then maybe he was arrested....who knows.