I'm guessing the person who reviewed it looked in the Immigration system. The really polite and informative message suggested it was not computer generated 😁
The 23rd Aug was the 90 days after my previous exit/entry (a trip to Pnomh Penh). When I reentered at Mae Sai, the Immigration system stored 28 Jun as my arrival date, so my new reporting date is September.
The TM47 system has no record of that arrival, as it doesn't pull your arrival date from the Immigration system.