Unable to low pass filter the signal

I am trying to use a basic example of a low pass filter as shown in the example: https://de.mathworks.com/help/signal/ref/lowpass.html

However, I change the frequency:

fs = 20e6;
t = 0:1/fs:1;
x = [1 2]*sin(2*pi*[70e3 300e3]'.*t) + randn(size(t))/10;
lowpass(x,100e3,fs)


The following is the response that I get:

The location of the peaks are at 68 kHz and 300 kHz. I am not sure why 300 kHz is not filtered. Any guidance will be helpful.

See the documentation at https://www.mathworks.com/help/signal/ref/lowpass.html and in particular the "Steepness" parameter: You didn't specify a steepness, so it is just using the default value of 0.85 which is a transition width of 15% of $$(f_{nyquist}-f_{pass})$$, or 15% of (10MHz-100KHz) = 1.485 MHz. You see this directly with the shape of your post-filtered noise.