Yes, I have installed hundreds of the Mesa shell holder and it is normal. It is not an issue and it will help protect the weapon. Remember, moisture is going to get behind that, so, if it was full size, that would be a larger area for moisture to get trapped behind and as we both know moisture is not your weapons friend. This is why it is designed with a protective strip and not a full size pad.
You have to take the shell holder off to remove the trigger plate assembly when cleaning the weapon. Each time you do, clean and oil the receiver on the side of the weapon the shell holder mounts. Make this a routine part of the cleaning of the weapon and there will never be an issue.
J.D. McGuire, Owner
AI&P Tactical, LLC
Remington Factory Certified
Law Enforcement Armorer