Thanks @dungengronovius, you suspected correctly! I did indeed install the plugin via WP CLI, and deactivating then activating the plugin through WordPress solved the problem – interesting. Thanks for the help!
Due to the time I have (or don’t have!) on my project, I have switched to using another plugin – Relevanssi. When I get time I will debug and take a look at the Search Everything plugin then report back here.
Nevermind, my mistake, works!
Nevermind, my mistake, works
Thanks @msaari, that’s perfect, just what I was looking for! Looks like a great plugin!