Bug can be reproduced easily, also on other Iphone models (tried Iphone 6s Plus and Iphone 6)
1. Uninstall the app
2. Re-install the App and sign in. All notes are shown in the App itself.
3. Try to search for a note with a keyword included in one of your note titles via Spotlight. No notes are found.
4. Add a new note
5. Search for a keyword included in the new note's title. This one is shown by spotlight, but not all older notes.All notes you create from now on, are found.