cheating or error in chess.com system
Or maybe you have a bad connection.
You don't connect directly to your opponent. Each side connects to the server separately. Your opponent can't hack your time.
https://support.chess.com/article/423-why-did-the-clock-times-suddenly-change-the-clocks-seem-broken
It can hack server and then your time but chess.com was tested by professionals so that isn't the case(in mist cases)