Please provide reproduction steps. For example on this forum the Testing Ground category is not visible to guests. https://community.nodebb.org/category/4/testing-ground. So if you browse this forum as a guest it doesn't show up on /recent and it won't be visible in the category dropdown on that page either.
Your quick reply (and example) made it easier for us to identify it as a layer 8 problem. Our subcategories did not have the expected rights. Sorry for raising this as a bug!