There are two things you can try.
On category pages you can only add a project if its a member of the category.
When ure inside your project, check the category work. Then in the work category page this project should be addable. But heres the problem if you have 'project arrows' activated, a project will only be able to have one category, not multiple.
Another thing: In pages you can add any projects with "+project". Maybe you want to replace your category page with a normal page.
Or maybe i should code it so that you can add any projects to a category page. I think I should code it like this.