Hi,
Thank you for the proper details. I tested all the results, and there is indeed 1 result there, that seemingly does not match any taxonomy.
All the other ones have a matching taxonomy term name, I have found in the Course editor screen. I have marked the matches on this screenshot: https://i.imgur.com/WeowXOn.png
The only item left is the “City & Guilds Level 3 IVQ Technician Diploma in Construction”, which does not seem to match anything. So I logged in via FTP, and added a debug code to see what exactly was matched there. The query works correctly, the content is not matched, however a taxonomy term was.
The taxonomy name is yst_prominent_words and the matched term name is exatly “drawing and surveying techniques 2”. This is connected to that post. It is not visible in the editor, so I’m guessing that this might be some sort of a leftover taxonomy from a plugin that you may have used before(?). The prefix “yst_” might indicate a YOAST plugin, maybe YOAST Seo? It could be something else of course.