Hi,
Thank you very much for the proper details, it helps me a lot.
1. The problem is, that many items does not belong to the main category, only assigned to the sub category: https://i.imgur.com/UxOYmjG.png
Because of that, there is no connection between that taxonomy term and the product, and wordpress returns a false value.
2. This is the same problem as the previous – since there is no connection between the product and the parent category, the filter has no way of knowing to exclude them. If an item is included within a sub category, it does not mean it is included within the parent category – as it can be explicitly unchecked.
It is because some items may not be represented by any subcategory, and a subcategory item may not be fully represented by a parent category.
3. This one seems to be working for me. I have tried searching ‘…’, when the search in title was enabled I got a result, when it was disabled I got no result.
Please note that this checkbox is not a ‘search only in title’ filter, but a ‘search in title’, meaning it turns on/off title search, and does not make the search exclusive to title only.