I can't pretend that I can understand people that want a search engine to omit terms they are searching for, though I have seen one or two such comments on past Kagi related submissions on HN. They are plain wrong! 😄
Of course, I have no insight into why more liberal interpretation of query terms involves removal of said terms. I could guess it has to do with how the search engines / indexes used behind the scenes behave and you might not have control over them.
In all it's faults, Google, which does this prominently, and which as far as I recall you use behind the scenes (along your other sources), at least highlights on the search result page for each result what terms are missing (crossed over words).
For me ideally the results should either
1) include all the search terms
2) highly boost those that contain all my search terms at the top (of course pinned/boosted domains can take precedence), and delineate those results that are missing some search them. Then I'd know where relevancy drops in the page.
In terms of what you can / should do, probably keep this issue somewhere on the back burner if neither of my suggestions are something that is technically simple or something that you'd like to implement. I've had these issues a couple of times from hundreds of searches, and probably will stay at the same ratio until your indexes grow substantially in size.