Thank you!
I disabled the plugin to do a few test and I noticed something very weird. Looks like there is an issue with the search results page itself, as for the same search query it always prints different results:
1. Search “hand”: https://i.imgur.com/YX9adLY.png
2. Search “hand”: https://i.imgur.com/ihzPZ9F.png
Unfortunately this is unrelated to the plugin, since it happens even when it is deactivated.