I have a teammate experiencing this original reported issue, inside Civil 3D 2018. When the viewport is rotated, the image shifts. When viewport is not rotated, the image behaves with zero shift.
I replied above and clarified that UCS rotate in viewport does not cause this original reported issue. I know this because UCS rotate is a regularly used workflow for me, with images.
So, I now think we can narrow this down to image type in a rotated viewport. I have done design on a long linear commuter rail project here in Northern California, and we have many UCS rotated viewports with geolocated imagery in the viewports, behaving perfectly. That behaving imagery is .tiff. Also, non-geolocated imagery behaves, in .jpg format.
In the original reported issue, that imagery behaving badly is geotiff (didn't get the exact file extension format). Also my issue is with .sid.
Possible solution: I converted the .sid image to .tiff, and the image in .tiff format now behaves perfectly in a rotated viewport.
Colby Phelps
Senior Civil Designer @ GHD
HP Z640 Solid State Intel(R) Xeon(R) 2.20Ghz, (2 processors);64GbRAM C3D 2020,Win 10 x64, Dynamo Studio user!