Some time ago, the Thai Wikipedian Potapt changed in few Wikipedia articles the spelling of Khwao Sinarin district to Khwao Sinrin. As I thought that all the articles were spelled according to the recommended English spellings as I found them in a list of the Royal Institute at first look this seemed like a mistake. But as I already knew Potapt as a very diligent contributor on the district and province articles in the Thai Wikipedia, there must have been more behind this.
It turned out that he (or she, don't know anything else than the username) referred in his change to the book "ชื่อจังหวัด อำเภอ/กิ่งอำเภอ ตำบล เขต และแขวง ไทย-อังกฤษ" published by the Department of Provincial Administration. It is the same one which is available online, and it contains the recommended English spelling down to subdistrict (Tambon) level. I mentioned it here before, and in fact Khwao Sinarin was one of the 135 romanizations which seemed wrong, contradict the normal RTGS rules, or contradict the above mentioned list of recommended spellings.
Another source mentioned by him was booklet named เขวาสินรินทร์, published in 2003 by the Royal Institute which gives the pronunciation of the district names, which for some complicate Sanskrit names is necessary even for Thai people. And there it also says it is pronounced "เขฺวา-สิน-ริน", Khwao Sin Rin. But even within DOPA one can find pages where it is spelled Sinrin as well as Sinarin, so no consistency there.
It is really a pity I never got any reply when I sent my list of corrections to DOPA after processing the complete book.