Why is my grace time resetting to 5 minutes?

If you are using the Laravel schedule monitor package you will need to specify the grace time via the console kernel (not the Oh Dear dashboard) as the sync command will replace it with the default 5 minutes if not specified.

// in app/Console/Kernel.php

protected function schedule(Schedule schedule)
{
   schedule->command('your-command')->daily()->graceTimeInMinutes(10);
}