Hello Mike,
Can you try attenuating the input to ensure you are not in compression? Spike also sets the attenuator and pre-amp automatically based on reference level. With your ref offset active, set the reference level to +40 to see if this affects the measurement. I am concerned that the signal is possible being compressed in the Spike software.
Having said that…
I performed a similar measurement on both the Spike software and version 2.18. I was able to get similar results on both pieces of software. I have attached the measurements. In this case, I used a signal generator to create a wideband signal with an output power of -40dBm. Seeing a channel power output reading of -40dBm on both software versions lets me know I have configured the software correctly. If you have a generator, can you try a similar test?
Regards,
A.J.