In common with a lot of "advanced search" on various sites, the implemented AdvancedSearch beta feature is not clear on how fields are combined. I guess contents in each field are or-ed together, and different fields are and-ed together. Some fields break this simple rule, for example «Not these words».

I would propose to add the operation in a parenthesis after the label for each field, and state somewhere else how the fields combine.

(The core problem is to make the implicit business rules that are obvious to the developers visible to the users.)