70-200L f4 IS fall off with FF

I purchased a new 70-200 f4L IS to use on a 5dII ( I had before another one that I used on a 30D that is dead due a downfall)

Detail and overall performance are good, but @f4 at 200mm I have an enourmous (for me) fall off of light; with the old one I never noticed that issue, but obviously I was on a cropped sensor...

What do you say ? "welcome in FF world" or should I worry for a possible defective exemplary ?

thank you

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Sandro