Create local backup of notes in Notes.app on macOS
You may have copied the wrong directory, depending on your operating system.

Newer operating systems (I believe El Capitan and above, but certainly Sierra and above) use this folder:

~/Library/Group Containers/group.com.apple.notes/


Without iCloud there should be no issue with disappearing notes, which you would have to work around, if you were signed into iCloud Notes.