I got caught out with this. I got my extension at Jomtien and thought I then should report 90 days after that. I, and those who advise me were incorrect. I had to pay overstay fine. It is 90 days from entry into country ... requires separate visit to immigration.
I got told the same at Jomtien this week when I went to report after returning from Australia...I have multi-entry non-b. No need to do TM30 if I haven’t changed address.
I noticed I was missing some comments in general. Then I read that if I click on the time stamp for the post the comments will appear. This has worked for me on my iPhone so worth a try.
I got fined for a one day overstay at Chiang Rai airport a while back --- and a stamp in my passport....rules might have changed since then. Not even an issue when applying for visas since though.