Hi LWDuncan,
Sorry for the inconvenience it may have caused.
For documents stored in SharePoint, you can open the document directly through the Office desktop client without downloading it.
And in fact, SharePoint theoretically will not automatically modify the data in the user's documents. Since you have stored the document in SharePoint, you can view the historical version of the document from SharePoint Online to check whether the document has no other edit records since your last edit. See View previous versions of Office files.
And we would like to collect some more information to further check it:
- Have you shared this workbook with others?
- Does the same problem also occur on other workbooks?
Appreciate your understanding.
Best regards,
Madoc