When the province of Bueng Kan was created in 2011, it quite quickly got the code 38 assigned by the Department of Provincial Administration - which was the obvious choice anyway as there was no other free number in the number range for the northeastern provinces. While there was no official new version of the TIS1099 national standard - the latest is still from 2005 - this list maintained by DOPA is in fact the current working copy of the national standard. The ISO 3166-2 standard, giving codes for the first-level subdivisions of all countries, uses the same codes prefixed with a TH-, but none of the updates since 2011 covered Thailand so far.
Today, ISO announced that they have changed the system of updating these codes named "Online Browsing Platform", instead of the newsletters it now has live updates on their website, to which one can subscribe - for free but one has to register. But actually thanks to the update scanner plugin in Firefox I will get notified whenever the page on Thailand will be updated, without having to register with ISO.
Anyhow, though none of the newsletters announced the code for Bueng Kan yet, with the change to the new platform the code for Bueng Kan is now also included, and without any surprise it is TH-38.