Problem getting an own category-feed

I am sorry, I don’t get it. I am referring to this link
Using option 3 it should be possible to have different feeds by categories. So I tried to get this done in my own podcast. And if I go to https://domain.tld/category/catname/feed I only get those that really are in the category catname. But if I go to https://domain.tld/category/catname/feed/mp3 it always is the whole list of episodes (both!)
Is there some setting I am missing, is my design flawed, what could it possibly be?