What happens if you get approved for the STV (evisa) after you've arrived on visa exempt? Just got the email and already got stamped in with 30 days. Does my STV 90day automatically cancel or does it mean that I have 90 days as opposed to the 30 I got stamped in with? Many thanks!
TLDR : Answer Summary
After being approved for the Special Tourist Visa (STV) while already in Thailand on a visa exemption, your STV does not retroactively cancel or change the initial 30-day stamp you received upon entry. Instead, you should keep the email confirmation of your STV approval, as it serves as your visa. When the 30 days are close to expiring, you need to exit and re-enter Thailand—either by air or land—and present the STV approval email for a new stamp allowing you to stay for the full 90 days granted by the STV.