Finally got round to checking this out, but I was unable to reproduce any problems from the save game no matter how I moved ships around
Here is a surefire way that I can reproduce the issue.
- Load a game with a 60x40 viewport
- Move a unit to the edge of said viewport, to where the viewport will auto-shift
- Let the viewport auto-shift, and there will be black terrain along both X-axis edges.
That works every time for me. It will go back to normal though after another viewport shift, so you will have alternating good and bad viewports.