OK, this is tricky and possibly not related to Relevanssi.
I have this category tree in WP:
The parent categories 1 and 2 are empty (no posts), they are used only as parents for the child categories.
I search with Relevanssi and then I filter the results by category. The URL looks like:
If I filter search by parent category 1, I get results from categories 11 and 12. Right.
But I recently created parent category 2, and when I filter search by that category, I'm not getting results from categories 21 and 22. They should be there, since they appear if I don't filter at all, and they appear if I filter their proper child category (21 or 22), but not when filtering cat 2.
If it is not Relevanssi-related but anyone knows any possible explanation, it'll be really useful. Having a precise search is my most important need.