I have this problem too, except it's like this:
- See a zombie.
- Machine-gun said zombie, e.g. with a Hunter.
- Kill the zombie, turning it into a reaper, then kill the reaper (with the same burst of machine-gun bullets).
- Get the death animation for the reaper, superimposed over a standing reaper. There is now an untargetable reaper there - it has an icon for "visible alien", and that icon centers the screen on it, but it doesn't highlight in red, and targeting cursors won't pop up when you mouse over it.
I haven't tried force-firing on the "second" reaper to see if bullets etc. will interact with it. It does become targetable as normal after it gets a chance to move... but since "letting a reaper move" = "getting a soldier turned into a zombie"...