Using a low-pass filter on PDM data giving me result that doesnt seem correct. What am I doing wrong?

I'm trying to convert PDM data into analog form by passing the data through a low-pass filter. My approach is fairly simple and matches the answer given in this Electrical Engineering Stack Exchnage thread: https://electronics.stackexchange.com/questions/521799/how-can-i-convert-an-audio-pdm-bitstream-into-analog-form

However, the result of the filtered PDM data looks like this:

Running an FFT doesnt work on this analog data. What could I be doing wrong?

I've tried a butter filter (various orders) and Cheby 2 filter and elliptic as well. For reference, the FFT looks like this and I'm expecting to see something in the ultrasound region: