AD9361 Rx Gain Issue

Hello

In our system 2 Rx channels are downconverted from RF frequency to 3 GHz IF frequency signals that enter the AD9361 device.

When 56 MHz BW is defined in this AD9361 device, the Rx signal spectrum analysis looks well like in the following picture (1 MHz sine signal is injected to the RF part):

When 16 MHz BW is defined in this AD9361 device, the Rx signal spectrum analysis looks strange like in the following picture (same 1 MHz sine signal is injected to the RF part):

What can be the reason for such strange signal shape in case of using BW that is smaller than 56 MHz?

Thanks