Steps to reproduce:

1. Open Word 2016 or later. Create a new blank document. Type any text. **File → Save As** → `original.docx`.
2. Verify mode 15: rename `original.docx` to `.zip`, open `word/settings.xml`, confirm `<w:compatSetting w:name="compatibilityMode" ... w:val="15"/>`. Rename back to `.docx`.
3. Open Telerik's WPF Demos sample in Microsoft Store (RichTextBox demo). Open `original.docx`.
4. Without making any edit, click **Save** (or File → Save As) → `roundtrip.docx`.
5. Rename `roundtrip.docx` to `.zip` and open `word/settings.xml`. The value is now `<w:compatSetting w:name="compatibilityMode" ... w:val="14"/>`.