I downloaded and unzipped the large data file, waveform.mat, using the link you provided. I see that the signal is complex, with fs=122.88 MHz and duration T=0.02 s. I ran the commands you suggested. I got the same result you got: the "bad" spectrum, consisting of a horizontal line.
The code you suggested using to make the spectrum included no options other than setting the sampling rate:
spectrum = dsp.SpectrumAnalyzer('SampleRate', Fs);
The screenshot you posted from the Wireless Waveform Generator (WWG) has many options on the left side. I suspect that the code that got exported did not include all the options that were set in the WWG tool. I am not a regular user of the Spectrum Analyzer Tool, and, as I mentioned before, I don't have the 5G toolbox and therefore do not have the WWG.
Let's compute the spectrum "by hand":
The screenshot of the good spectrum, which you posted, and the bad spectrum both show RBW (resolution band width)=120 kHz. Therefore I will compute the spectrum with Welch's method, and I will use a Hann window of width Nw=fs/fRBW=1024 points. I choose Welch's method and a Hann window because the Signal Analyzer appears to use these by default.
waveform=load('waveform');
[pxx,f]=pwelch(x,hann(Nw,'periodic'),0,Nw,fs,'centered','power');
pdBm=db(pxx*1e3,'power');
ylabel('dBm'); xlabel('Frequency (MHz)'); grid on; xlim([-62,62]);
The code above makes the figure below. This is very much like the good spectrum you posted, but less noisy. The height of the center plateau is the same (-30 dBm), the low values on the sides are similar (<-60 dBm), the frequency range is the same (+- 61 MHz), and the plateau cutoff frequencies are very similar (about +-48 MHz).
