Files disappear from nextcloud desktop

Hi folks,

In our company we have a department with 4 users. These users have a shared folder that they access with the Nextcloud desktop application. A few days ago I was told that some files in this folder had disappeared from random subfolders. I couldn’t find the reason, so I had to recover those files from the backup. The next day I was told again that more random files were lost and I had to recover them from backup again.
I have tried to find out the cause of the deletions, but have not found it. If you have any clues I would appreciate it.

I would like to understand the way the Nextcloud desktop application works :
User 1: creates the folder A and inside it the file A.txt.
User 2: creates folder B and inside it file B.txt. It then synchronizes the shared folder
User 1: synchronizes the shared folder at this point.
In this scenario would file B.txt be deleted?

Kind Regards