[not_so_young_greg]

Yes, my bad, the module works.

I have figured out my problem. It might help somebody so I will try to describe it.
I moved from 4.5.4 to 4.6.5. I did not update but imported products into a new installation, using x-cart export-import feature. It looked ok.

But most of my products have variants so availability is based on variants, not on general "available quantity" (you know, the box on product details page which gets disabled once you create variants). Some of my products imported with 0 as general available quantity (why - I do not know). It is hard to notice - the "available quantity" box does not show anything if a product has variants. Works ok though.
The only way to check it is to either disable variants or look at the database.

Since the recent Refine Filters version does not check for "avail" field in the "variants" table but checks for "avail" field in the "products" table, I got all weird results - and I mean weird (it was not the case with the former version because it listed all products including out of stock items).

I had to write a small script to update "avail" field if variants available are > 0. Now it seems to work ok.

Hope it helps.