Sync only runs when Obsidian is in the foreground. Closing the app quickly after adding text may prevent it from syncing all the changes, which can lead to sync conflicts if you then edit the file on another device. The Sync status icon lets you know when Sync is working or has finished. It is hidden in the right sidebar but can be made always visible with a CSS snippet as in [Mobile] Make Sync icon always visible.
The duplication already has a bug report, so I’ll merge yours into that thread.