[Python] resample_poly from scipy.signal gives all-zero result on pi3b

I am building an audio signal detector and have a basic version working on PC, but cannot get it to work on my target Raspberry Pi 3B. I have narrowed the problem down to down-sampling using resample_poly (to reduce CPU load). I have prepared a really simple code sample:

import numpy as np
from scipy.signal import resample_poly

audio_data = np.array([ 43, 58, 67, 88, 89, 99, 121, 113, 88, 69])
sample_rate = 4000
file_rate = 16000
print(f"Before: {audio_data}")
audio_data = resample_poly(audio_data, sample_rate, file_rate)
print(f"After {audio_data}")


On PC, I get: After [ 28.66957128 103.77145182 80.49835032] On Pi, I get: After [0. 0. 0.]

This is running on numpy-2.0.2 and scipy-1.13.1. The only older version I can get to run is numpy-1.23.0 scipy-1.8.1 with the same result. Other versions are not available as wheels and would not build on my pi, or failed on import in Python. Driving me totally mad just now!!!

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