When I plot negative values using mne.viz.plot_topomap, I get spurious positive values near the boundary:

import numpy as np
import mne


m = mne.channels.read_montage("biosemi64")

info = mne.create_info(m.ch_names[:-3], sfreq=512, ch_types="eeg", montage=m)
np.random.seed(1)
values = np.random.randint(-100, 0, 64)
mne.viz.plot_topomap(values, info)

This is likely an interpolation artifact, with possibly serious consequences when interpreting scalp maps. Or maybe it is specific to the biosemi montages.