Microsoft Office applications (Word, Excel, and PowerPoint) have a bug that they can’t properly load a file within a WebDAV share when its name has a special character (e.g., an umlaut like “ö”). The files are successfully saved/encrypted so there is no “data loss”. But if you re-open the file, the Office application will load an old version (probably from its internal cache). If you close the file and rename it, the Office application will correctly load the file.
The workaround is not to use special characters in filenames (only applicable to Word/Excel/PowerPoint files).
Either Microsoft is going to fix this or this issue should be solved when we provide the virtual drive using some other technology than WebDAV in future versions. The progress is documented on GitHub.