I’ve noticed something similar using SoapySDR and an RSP1A. I get a high noise floor and strong signals 250MHz to either side of the baseband, and then it drops off significantly – This seems to be irrespective of the sample rate I’m using, i.e. 500kSps through to 8MSps.
You can clearly see the drop off at either edge of the 500kHz around the baseband, and then you can see it roll off at the end of the full sample space.
I’ve attached an example screenshot from GQRX. The sample rate is set to 4MS/s.