Not 100% certain, but I think Jekyll only treats subfolders as “categories” when they’re placed in _posts/
.
└── _posts
└── work
└── another-category
You’re doing it the other way around by sticking a _posts
folder within the root category folders. If you want to stick with that you can manually assign a category name to the posts with YAML Front Matter.
Adding category: work
to your .md
inside of /work/_posts/
should do the trick. You could also assign it to all the files in that folder using front matter defaults in your _config.yml
.