Were these notes made on the same phone? Are your notes being synced to your Samsung Account? Last year (before they deprecated the app and went all-in for Samsung Notes) there was an issue after an update with lost notes, and unfortunately, there was no fix if the notes weren't backed up physically somewhere else. If the notes you're missing are from that timeframe, that might have been what happened.
Also, are you sure the notes were saved? Sometimes if a note is left open for too long and the system closes the background process, the note might be lost if it wasn't saved.