Seems like there was a typo in @misaka4e21 commit: UX: Improve category filtering and include subcategories · discourse/discourse@6db623e · GitHub
in_category_and_categories instead of in_category_and_subcategories
I fixed it in:
https://github.com/discourse/discourse/pull/6439
If you are around @misaka4e21 can you confirm it was your initial intent? Otherwise I will merge this at the end of the day.