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Hi Alex,
I could replicate the pb and start to investigate.
You have a strong transient artifact in your data (stim artifact I suspect)
and this affects the conditioning of the mixing matrix. So when computing
the pinv here: https://github.com/mne-tools/mne-python/blob/master/mne/preprocessing/ica.py#L677
the mixing is not numerically the inverse of the unmixing matrix.
I don't know exactly how to fix this but that's a starting point.
He someone can look into this it's great.
Alex