Enblend 4.2 generates stargate-like output for large pano

I am stitching a 1.1GPix panorama from 81 photos. I am observing that, repeatedly, the output panorama has a bizarre stargate/LSD like effect approximately two thirds of the way in (in order of photographs passed to it). I've attached a PNG showing the effect.

I've attempted to output into PNG and into TIFF, both with the same issue. I haven't tried output into JPEG, because the panorama is bigger than what JPEG can hold.

Enblend was invoked from Hugin. The only additional flag is "--fine-mask".

Logs show nothing suspicious, except perhaps this warning:
"""
[...]

Blending images...
enblend: warning: option "--fine-mask" combined with option "--primary-seam-generator=graphcut"
enblend: warning: incompatible with mask optimization,
enblend: note: defaulting to no optimization
enblend: info: loading next image: 0000.tif 1/1
enblend: info: loading next image: 0001.tif 1/1
enblend: info: loading next image: 0002.tif 1/1
enblend: info: loading next image: 0003.tif 1/1

[...]

enblend: info: loading next image: 0106.tif 1/1
enblend: info: writing final output
Updating metadata...
    1 image files updated
"""

Enblend:
$ dpkg-query -W enblend
enblend 4.2-2
$ lsb_release -d
Description: Debian GNU/Linux unstable (sid)

Machine:
Intel(R) Xeon(R) CPU E3-1231 v3 @ 3.40GHz
32GB (4x8GB) RAM