Can't Select Entities at Non-Zero Elevation

We have a civil engineering file with contours (2D polylines at their real-world elevation). Twice now, for unknown reasons, the drawing gets to a point where we're unable to select any entities unless they are at 0 elevation.

I tried resetting the UCS, audited the file, etc. but nothing seems to help.

We are running a third-party civil package, so there may be something going on there, but I wanted to see if anyone had run into this or had any ideas?

Thanks,

TJ

let's move this out to 'bugs and quirks' and see if anybody can answer it there (esp as it's not an AutoCAD Civil product :wink: )