subtractSky1d applied to spectrograph=3 produces excessive noise

rhl reports:

With this how-to-repeat, these plots are pretty-much a smoking gun for a problem with spectrograph != 1:

dataId.update(spectrograph=1)
spec = butler.get('pfsArm', dataId)
pfsConfig = butler.get("pfsConfig", dataId)
​
from pfs.drp.stella.subtractSky1d import subtractSky1d
sky1d = butler.get("sky1d", dataId)
subtractSky1d(spec, pfsConfig, sky1d)
​
spec.flux[(spec.mask & ~spec.flags["REFLINE"]) != 0x0] = np.NaN
​
fig = 1; plt.close(fig); fig = plt.figure(fig)
spec.plot(spec.fiberId[100:101], figure=fig, show=False)
plt.ylim(-1000, 1000)
​
plt.title(dataId);