[ see https://bugs.launchpad.net/ubuntu/+source/eog/+bug/1746146 ]

I have been using Hugin to assemble panoramas, but until this one, they were all horizontal (all photos to be assembled right to left).

This one contained four images vertically oriented, so to line them up, I copied them and rotated them 90 degrees counterclockwise with eog and saved through eog, and then pointed Hugin at the rotated images (Possibly Hugin could make a vertical panorama without my doing this, but I didn't try...)

After the panorama image was complete, I copied it and rotated it into a vertical orientation again (90 degrees clockwise) and saved through eog.

Imagine my surprise, when I looked at the final image, and saw a strip of pixels on the right that didn't look right.

Upon closer inspection, I found that they seemed to match the pixels on the left edge.

When rotating, the image appears correct. It is only after saving, and re-invoking eog on the image that the problem is visible.

It is not a display problem, as the copied/moved pixel columns show up when the image is shown with ImageMagick, and GIMP.

With GIMP, by blowing up the display to 400%, and watching the pixel coordinates as I move my mouse from the left to the right over the false pixel columns, it appears that the number of pixel columns copied or moved is 7.

Upon further research, When I have GIMP blow up both images equally, it seems clear that the pixel columns are being cut off from the left side of the image, and attached to the right side.
(In the attached images, if you look at the "C" on the left (about 75% of the way down, at the edge), you will see that it lines up almost exactly with the edge of the image, yet in the horizontal image, there is a gap between the edge and the "C" (that appears also to be about 7 pixels wide))

Also:
If you look at the right edge of the image, opposite to the "C", the rest of the "C" as it fades out can be seen - but the dark area is to the left, not the right.

This appears to indicate that the columns of pixels that are transferred are brought over in reversed order (mirrored).

(If the chopped-off gap-leading-to-"C" were not reversed, the dark area would be on the right fading to the left as it leads from the "C" into the gap).

I hope that I have described clearly enough, what I am seeing...
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scott@scott-Asus-M2N68-AM-PLUS:~$ uname -a
Linux scott-Asus-M2N68-AM-PLUS 4.13.0-32-generic #35-Ubuntu SMP Thu Jan 25 09:13:46 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
scott@scott-Asus-M2N68-AM-PLUS:~$ lsb_release -dsc
Ubuntu 17.10
artful
scott@scott-Asus-M2N68-AM-PLUS:~$ echo $DESKTOP_SESSION
QLubuntu
scott@scott-Asus-M2N68-AM-PLUS:~$ eog --version
GNOME Image Viewer 3.26.1