The docs are clear: "If a note for that contact already exists, the plugin will not modify it and will only insert the link. This means you can safely make manual edits to contact notes without worrying about them being overwritten."
Since there is no two-way sync, the locally modified notes are never sent to Google. This makes the Google's version act as the source of truth.
Unless you're planning a two-way sync, it would be useful to have a command which pulls the Google's version for the existing note and gives a diff of the fields.