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.
I dragged both folders from user library unto a external drive to back them up:
group.com.apple.notes
and
group.com.apple.notes.import from in the user library
You can create a test user using these folders into that user library and all will reappear (but not in another user) they are signed.
The reason I had to do this was to fix sync issues, so after I made a backup I went too the system prefs, Apple ID account, chose Manage (iCloud storage) and erased all the notes across all devices.
After dragging back the two folders into the user library everything went back to normal and started syncing again. It was the only way.
You also need a good wifi connection and internet plan for this if you have tons of gigs of notes with photos, videos etc, I have 82GB: Notes replaced the rest for me.
Apple's official recommendation to backup notes is to save them in a text/document file or export as PDF. From the Apple support article, Archive or make copies of the information you store in iCloud - Notes and Voice Memos
Notes and Voice Memos
Make a copy of a note on your Mac or at iCloud.com:
Open the Notes app and select the note that you want.
Copy and paste the text into a document on your computer, such as a Pages or TextEdit document.
Save the document to your computer.
You can also export a note. Just select the note, click File > Export as PDF, and choose a location.
The approach is a bit clumsy and doesn't let you backup all the various notes in one go.