The saved USN cursor also needs to be associated with the notebook and tag scope that produced it. If a user expands the configured scope, previously excluded notes may not receive new updates and therefore never appear in incremental chunks after the existing cursor.

I would treat a scope change as a reconciliation event, with a test that adds a notebook containing unchanged notes. The opposing test narrows the scope and checks removals only after a complete listing succeeds. That exercises configuration changes separately from the update and deletion events your fake client already covers.