a highlighted section without any notes associated with it is currently treated the same as a highlighted section with comments. In case a notebook is assigned in the Highlights setting, highlights (without notes) get assigned to that notebook. This fills the notebook up without notes.
Suggestion: differentiate between notes and highlights. This means highlights without notes get assigned to a standard 'Highlights Notebook' and highlights with notes (which are actually notes) can be assigned as it is currently implemented.