Hi,
Thank you very much for all the details, it helped a lot.
It was actually a yet unknown bug related to a very specific configuration causing the problem. The issue was related to searching the post IDs, along with the “AND with exact keyword matches” logic + the stop words in combination.
What happened is, that the plugin tried to get “AND” matches from both the indexed sources and the IDs separately, ending up with some results being excluded and reverting back to the secondary logic, as if the primary was never considered.
I have made a few direct changes to the main search code to address that, I will make sure to include that in the upcoming update, so you don’t have to worry about it.
I have also made a few changes on the configuration, adjusted the keyword logic, and also added the stop-words as whole keyword exceptions so when people type it in they are being ignored (as they are not indexed). Now you should get much more relevant matches as expected.