Have you investigated this further
@amirm ? It appears there are only two frequency steps in the AP's sweep at the "notch", 45Hz and 47Hz. It's clearly in the analog domain at that point and there is no way a notch could be that deep and be ruler flat either side within 1Hz.
Perhaps the device is muting the output due to the absence of what it determines is a valid load for some reason. What is the load on the device on those FR sweeps? And the duration, is it a chirp or a slow sweep?
I can see no other way you could go from 0dB to -60dB in 1Hz.