With the attached terragen 3 file, if I load it and going to the Water area and select Lake 01. If I then change the water level by clicking the up button, that takes the level to 531, I decided that was a bit deep and from the edit menu select Undo to revert that change. The 3d preview goes black but the change appears to have been reverted, the level is back at 331. If you do a render the render matches the preview, everything's vanished! Then if you tilt the camera down you'll see that the planet is rather far away.
If you put the level back to 331 using the down arrow rather than undo the behaviour is a lot more sensible. Maybe there's something strange in the rest of my model but the behaviour is a little unexpected and hard to put back (except by reverting to the saved copy). I tried replicating with a starting from scratch approach but so far haven't got the bug to appear.
Terragen 3.3.04.0