I’d need to see the 3dm to know for sure, it could be the mapping method used or the axis you rotated the texture in. Post it and I’ll take a look. You can use the Export command to save out just the current selection as it’s own 3dm.
This looks like a bug when the render mesh has very few polygons in it. I filed it as https://mcneel.myjetbrains.com/youtrack/issue/RH-57761 for future reference. It doesn’t happen in Raytraced mode if that is an option for you as well.
A workaround for Rhino Render will be to make a custom render mesh with more polygons for the object prior to using Unwrap. Unwrap will then use that new render mesh preventing the issue.