Many thanks for letting me know. It does seem very arbitrary though. They never give any specific period and there is really no rule on how many visa exempts would be issued for one person.
After you had been denied entry at DM in 2019, did you finally manage to get back in? If so, how? You said that you bought a ticket to go to Chiang Mai on July 2, so what actually happened between then and now? Do you mean you have been outside the country for all these years?