Vibrations and low rpm at stoplight

Mazda 2 2008 drops from 750 RPM to 550 RPM when stopping in Drive and pressing the brake, causing the car to shake.

I have already replaced the upper right engine mount, but the issue persists. I also tried cleaning and replacing the throttle body, but the car still vibrates.

Someone told me that this RPM drop is normal and that it's a fuel-saving feature in Mazda. Is this true? Is 550 rpm normal?

What can I do to fix the vibrations at a stoplight?
 
Does this happen when you are in neutral as well or only in drive?
 
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