Why is my LC filter Bode flattening out to 20dB/dec in my Simscape simulation?
There are many ways to plot the response of a circuit with simscape, the question is what method did you use

Using the bode block it can be seen above the natural frequency the rolloff is 40dB/decade.

There is a high chance that if you manually did a frequency sweep to a matlab workspace variable and then post-process the data, that you did not select a suitable sampling rate to reconstruct the data. Likewise the simscape reference block include a filter response as well.