That typically happens when you leave the app before Sync has synced the latest change. Obsidian autosaves every few seconds, and I believe Sync syncs any available changes when that happens. So in your example what probably happened is a sync at the point where you had typed “a note on”, and then in the short span of time before the next sync you finished the sentence and left the app.
The Sync status icon in the right sidebar tells you when sync is syncing. There is a feature request to make the Sync icon always visible so you know when you can safely leave the app. As a workaround in the meantime there is a snippet that does it linked in the request. [Feature] Make Sync icon always visible