Is it common for the online system to incorrectly remind me of my TM47 reporting date after re-entering Thailand?

Yes, it is common to receive incorrect reminders from the online system regarding the TM47 reporting. This happens because the system does not recognize when you leave and re-enter Thailand, so the counting of your days resets only when you calculate your entry date as day one. It's advisable to track your own reporting dates and expect to report in person, as the systems may not be synchronized.