Hello,

This issue is typically due to lag or latency between your connection and the game server. When your opponent makes a move, there might be a slight delay in transmitting that information to the server, causing the clock to appear to decrease and then add seconds back. It's not that time is being added; rather, the clock is synchronizing with the actual time once the connection catches up.

If this happens frequently, it might be worth checking the stability of your internet connection. In some cases, playing on a more stable or higher-quality network can reduce these types of issues.

Best regards.
xtreme-ball