Noticed notes were properly being created, updated and deleted on PC, but not on iPhone. Any iPhone action taken within the app would not persist, despite sync symbol firing and notes appearing to save. No careful means of waiting/syncing resulted in note modification persistence.
Fix that worked for me on my iPhone:
Delete everything in Trash
Uninstall App (Copy any notes not synced elsewhere)
Reinstall App
After doing that, can successfully create, update, and delete notes on iPhone.