Filtered signal's frequency response is not correct

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Ryan Takatsuka 2018-11-26

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https://ww2.mathworks.cn/matlabcentral/answers/432107-filtered-signal-s-frequency-response-is-not-correct

test.m

I am trying to pass a pure noise signal through a filter, but the frequency response is not what I expected. I have attached the sample script I am using to filter the signal.

I created a digital FIR filter as shown:

I applied this to flat noise and expected to see abount -120 dB of attenuation at higher frequencies; however, I get the following frequency response:

A few things stand out as incorrect:

Why isn't the amplitude flat in the stopband of the filter (~1000 Hz to ~64000 Hz)? I would have expected the orange filtered signal response (the 2nd figure) to match the filter response in the first figure.
The attentuation in the orange filtered signal only reaches about -60 dB, when it should reach -120 dB. Why is this?
I ran a similar test using Simulink and the spectrum analyzer produced the correct frequency response for the signal. Why is this different that using a MATLAB script?