You can see how the three outer vacuum engines are greatly over expanded giving you a collapse of their exhaust after exiting the bell and then the Mach diamonds.

Where the inner sea level engines are a much closer expansion match and don't really form visible Mach diamonds in view of the camera.(Edit: you can faintly see them. But the compression rebound that forms the visible diamond is much less)

It's quite interesting that they can light and run those vacuum engines on the ground, usually the combustion instability from an over expanded exhaust at sea level results in a hard start.