Data Sync Issue after server is online

Dear All,

I am using latest Version of Nextcloud on Ubuntu. i have one Query and request for suggestion. my use case is as bellow:

when Nextcloud Server is down or under maintenance of server or Hardware, in between if there are incremental changes in user data on local system, are being replaced by by Nextcloud server Data when server comes up.

in detail:

Lets say, server is down for 6 Hrs. in between user A was working on this Laptop with local synced folder and did so many changes but when Nextcloud server is back, all changes done by user A in his local folder get replaced by data on Nextcloud Server. This mean his all work done for 6 Hrs is gone.

How to prevent this.