I was a Udemy instructor for ~10 years selling tech courses but focused more on delivering courses through my own site for the last ~5-6 years.
Something never felt right with how Udemy promoted courses. I used to have a top selling course there, selling thousands of copies a month and now it gets basically no sales but it's still one of the highest rated courses in that niche on their platform. It's just no longer ranked or promoted by Udemy, for years.
I have no evidence of this but my personal opinion is their ranking is probably not fully automated and they have special offers and deals with certain instructors and if you're not a part of this club, oh well.
Again, it's all speculation but I can only go by what my numbers are. They were small scale life changing and now nothing but the quality of the courses I produced didn't change. It doesn't make sense. Of course it could be one big coincidence too, but this has been tracked and analyzed over years.