Have you tried using other than Sigview SW or must you use it ?
You can process it with any software, for example you can use LabVIEW or any tool for FFT.
The FFT is just highlight what is wrong with the data. It shows that the noise is not white.
It has significant spikes at some frequencies and looks like intermodulation distortion.
But you can see that the noise is much higher even by opening file in any text viewer.
You will found that the new firmware produce noise with values 126, 127, 128, 129, 130.
The old firmware produce noise with values 128, 129, 130.
The difference is pretty clean and obvious.
I have some assumptions about the root of cause for this issue:
1) The new firmware performs ADC data processing which add distortions.
2) The new firmware trying to perform some actions in parallel with ADC capture. It leads to RF interference.
3) The new firmware write corrupted BIN file.
But in any case this is a bug and should be fixed.