I started a server with my friends. At first backpacks work fine but hours later the backpacks start sharing the items with other guys and some backpacks just dont have the items it had previously.
Version 1.15.2

I assume you are, but I have to ask are you using the latest version 1.0.8?
Could you maybe send me some of the server logs, so I can if there's an error thrown somewhere?
Did this start to happen after a server restart?

If I don't have more info I can't fix the problem.
Feel free to reopen.