@amris it's been like this for decades, it never really worked and probably never will. The idea of a "main category" is just stupid because, as you pointed, it is arbitrary. When you perform a category search, it will only look for main categories. If you want to include a specific subcategory you have to use the Browse page, and doing so will only search inside subcategories, so torrents with "trans" as main category will be ignored.
These are the kinds of problems you create by making everything an arbitrary choice and not accounting for people's decision making and search behaviors in the process.
It's such a simple solution, having main/sub categories have the same weight, in other words, if the user searches for a category the site will look in both main and sub... In fact it's so simple that if it's not done yet it's because they don't care.
