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Title: Excel (Microsoft 365 on Windows 11) corrupts local files created in previous Microsoft 365 versions after first re-save

Environment:

- OS: Windows 11 (fully updated)

- App: Excel, Microsoft 365 (current version/build — please advise how to pull exact build if needed)

- File type: .xlsx, created locally in a previous version of Microsoft 365 Excel

- File location: Local drive (same folder throughout; file is not moved or renamed)

Issue summary:

Excel files created in an earlier Microsoft 365 version become corrupted after being opened, edited, and saved once in the current Microsoft 365 version. The file opens fine the first time, but on the next open attempt from the same location it fails with a file format mismatch error.

Steps to reproduce:

1. Take an .xlsx file that was created and saved in a previous version of Microsoft 365 Excel.

2. Open the file in the current Microsoft 365 Excel on Windows 11. The file opens normally.

3. Do your work, save, and close the file.

4. Attempt to reopen the same file from the same location.

Expected result:

The file opens normally, as it did the first time.

Actual result:

The file fails to open and Excel displays the following error:

"This workbook couldn't be opened because the file format may not be matching with the file extension. To resolve this issue, try restoring your file to a previous version. Click the link below for step-by-step instructions."

The error dialog includes "Restore a previous version" and "Give Feedback to Microsoft" links.

Additional details:

- This is reproducible and has affected multiple files, not a one-off.

- The file is not open in another application or locked by OneDrive sync at the time of reopening.

- Restoring a previous version recovers the file, but any edits made in the current version are lost — so this is causing real data loss.

Questions for the community / Microsoft:

1. Is this a known issue with the current Microsoft 365 Excel build on Windows 11 when handling files created in earlier builds?

2. Is there a recommended workaround (e.g., Save As to a new file, converting the file format, or rolling back the Office build) until a fix is released?

3. How can I capture logs or file samples to help Microsoft diagnose this?

Any guidance would be appreciated. Happy to provide additional repro details.