@Ant****
it's not necessarily an anomaly :/ but certainly a white elephant :P ;)
Bangkok will tell people on O-A year entry stamps that you can't get a marriage extension for the first extension and you have to do at least ONE extension for the reason the visa was issued. That means, seeing as O-A visas aren't issued for marriage but for being over 50 you need to do one extension based on retirement.
However different offices seem to have a different interpretation of it and will let people On O-A year entry stamps go right onto a yearly extension based on marriage.
If you're on a year O-A visa entry the only rock solid advice I can give is CHECK with your immigration office well before you need to apply for your extension to see what they will and won't allow.