You mention Vue6, but I've never seen this in Vue6, so maybe this was a typo...
But in Vue7, by default,  the camera has a "locked" altitude, it is set to 1.8m from the object under the camera.
If there is only an infinite plane below the camera the camera will be positioned at 1.8m above it. If there is a terrain beneath it, the camera will jump up to be at 1.8m above the terrain.
To prevent this, go to the object properties panel (upper right), with the camera selected, and unlock the camera altitude by pressing the padlock icon in this panel.