Don, I set up a game as you described and observed the following:
- When removing the trenches, one trench element remained and I wasn't able to remove it
- The build points didn't fully increase, it stopped at 19
- When laying again I was able to lay one, sometimes two, trenches more than should be (with build points at zero that is)
If you need more detailed info or specific setup, or savegame file, etc., let me know.
I am not sure though, whether the left over trenches are treated as such, or are just graphic glitches, like the green junk.
I attached two image files, but they are quite ugly, because of upload filesize limits...
