The implementation is not necessarily wrong, even if it may not be correct choice for your use case. Libraries that implement filters like this usually have a parameter or something where you can specify how to handle the border of the data.
What your LLM implemented is equivalent to scipy.ndimage.convolve with mode="nearest", and you wanted what numpy.convolve does with mode="valid"
https://docs.scipy.org/doc/scipy/reference/generated/scipy.ndimage.convolve.html
https://numpy.org/devdocs/reference/generated/numpy.convolve.html
