The contents of the excel file are corrupted

Currently I am in trouble because I do not know the cause.

There is an excel file placed on NC. We open and edit them via WebDAV on Windows. The user mounts NC on Windows and opens it in an excel application. We do not edit excel files using Collabora Online.

When a user opens a file, the contents of the excel file may be corrupted. The font has changed, the cell width has expanded, the image size of the image file has expanded, the printable area has changed, and so on. Excel formatting has changed even though no one is editing the file.
The degree of their damage does not seem to be the destruction by human operation.

This has not happened with the files that the user has on their local HDD.
This suddenly started to occur about two months ago. Can you imagine what caused it?

NC 19.0.4, CentOS 7.8, MariaDB 10.2.32, PHP 7.4.12