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ADL5375 noise floor


I'm looking into the noise density of the ADL5375 IQ modulator and I noticed that it degrades when the LO frequency goes above approximately 4 GHz. Up to 4 GHz noise density is around -155 dBm/Hz as expected but with LO frequency of 5 GHz it degrades by 10 dB as it can be seen in the attached figure. Moreover, the noise floor degrades not only around the LO frequency but also at the low frequencies. 

Is this something I have to live with or is there a way to improve the performance? According to the datasheet noise density shouldn't degrade that much with the frequency so I wouldn't expect this to happen.

In the figure white shows noise floor with LO at 3 GHz and red shows noise floor with LO at 5 GHz. LO power is 0 dBm in both cases. 

I also nulled the LO leakage in the 5 GHz case but it doesn't make any difference.  Also. I checked the LO source and the noise density is the same at both 3 GHz and 5 GHz.