Hi again,
This is amazing: I have been wondering about this category sitemap for weeks and just after posting my question, I noticed I had the same problem with one of my pages.
Instead of ‘automatic detection’ in the advanced page options, I chose ‘always include” and the page appeared in the sitemap.
I did the same for all my categories and the category sitemap works now.
I have no idea what is wrong with the automatic detection though. I am going to try to inform Joost about this.