Thank you for your patience as we investigated and identified the root cause - we found and fixed two issues with Thai locale flows in Unity IAP.
Version 2.2.7 (not yet released) will completely fix this issue.
The [first fix in 2.2.6]( Unity IAP package 4.12.2 is now available page-2#post-6745459) addresses non-Subscription Products, and only partly addresses this issue.
These fixes improves the stability of Unity IAP. Thank you very much for the clear reports.