OK, I have the contact sheet open on my main monitor and the preview window open on my second monitor. No images are selected. The preview window displays the first image from the contact sheet.
I click on, say, the first image to select it and then I command+click on subsequent images to make a non-contiguous selection. If I scroll the sheet down and then command+click on another image the contact sheet jumps back up to the first image, which is currently showing in the preview window. (Whilst making further selections, the image in the preview window remains the same).
I guess a possible solution would be for the preview window to always change to the last image selected, even if it is part of a multi-image selection.
Hope this helps.
Cheers,
Paul