I have checked the results for that phrase, and it seems like that all of them match all of the keywords, either within the title or the content. The plugin reports more results, so I think this is also matched, but not displayed, as by default the display limit is set to 10.
You can increase this limit under here: https://i.imgur.com/SGZz2Di.png
That should do the trick.
That configuration looks okay to me. Can you please add temporary FTP and back-end details? I would like to debug the query and check the configuration to see if everything is all right. Thank you!
Thank you very much for the details, I have made an adjustment to the search.
I have noticed that you have multiple fields to get the keywords from, in that case, usually using the index table engine is a better option. Since there was site access, I have configured and enabled the index table engine for that search bar. You should now see much more relevant results, including the missing one.