Maybe we talking 2 different topics here. You Always need to register if you’re not on a tourist visa. In fact you always need to get reported but I have never gotten any problems if I didn’t get reported while I was on a tourist visa applying for an extension
i got extension in Phuket amd Chiang Mai without any problems. I guess it comes down if you need to show them TM 30 or not. And that has never been the case for me if I was just doing an extension