What does the amber "Update available" label in the note preview mean?

It means you've written more on that page on your Supernote since the app last saved its copy, so the preview is slightly behind. The app deliberately does NOT auto-reload (re-converting can take ~15 seconds and you might be mid-edit), so it shows this quiet cue instead. Click it (or the circular refresh arrow at the top of the preview) to pull the latest version on demand. The label clears once it loads.