The path operator performs the subquery on the FULL FILE PATH which includes file names. If you have a file(s) that share some similarity in their titles to the desired restricted folder, they’re going to show up in the search. I’ll attach a screenshot to make this crystal clear.
When you’ve got 10k+ notes split across 50 notebooks (aka folders) this starts to become a real issue.
There doesn’t appear to be any way to restrict your search to a single folder. I don’t know how it’s possible that I’m seemingly the first person to run up against this limitation. I’ve had to start working around it by autotagging my notes with delimited parts of folder/subfolders. It’s a bit of a hack.