Issue starts from here.
Current REST API doesn't have the feature - updating content of the paragraph.
We may have to check UI issue (or side-effect, if any) when implementing CRUD to paragraph via REST API.
Before going further, there's a way to minimize potential side-effect, providing initial paragraphs when creating new notebook via REST API.
Users couldn't see new notebook before persisted so I believe it doesn't make any noise with UI interaction.