As your description, my understanding is that the changes of document from Outlook cannot synced to SharePoint site library.
I could reproduce this issue after I synced a SharePoint Library to Outlook and made some changes in a document of the library from Outlook.
For solving this issue, please double-click the document to open the document again. Then an Edit Offline dialog box will be shown. Click Update to update the server with offline changes.
Here is a similar blog for your reference: