At first short courses were approved but very quickly most embassys started looking for a course that matched the length of stay... 180 days. They were also many reported cases of courses less than 3 months resulting in a tourist visa.
I run a muay thai gym and we have learned how to present a course in a way that it's always accepted. Done 90 now.
One of my customers ( I have a muay thai gym) had a terrible history. Had been warned at the border twice. HCM gave him the DTV and Said, good this visa is made for people like you.