Unexpected peaks in power density following downsampling and filtering
2 comments:

First, scipy signal decimate already low-pass filters the signal. It first filters the signal and then subsamples. So there is no need to pre-filter your signal before decimation. In your case, the signal is filtered at 125 Hz, which is half the resulting sampling frequency. (decimate doc here)

Second, a Butterworth filter is not an ideal filter. This filter will have a 3dB attenuation at the cutoff frequency, no matter the order of the filter. So you are expected to have components remaining between 100 Hz and 125 Hz.

If you want nothing detectable above 100 Hz, then design a filter with stop-band at 100 Hz and sufficient attenuation. This will result in a cutoff frequency below 100 Hz.

Depending on your requirements, an elliptic or Chebyshev filter will result in a narrower transition than a Butterworth of the same order.