files from multiple artboards have the same thumbnail

hello,

I like to work within one Illustrator file with multiple artboards and use save as to generate multiple Illustrator files. These files have all the same embedded thumbnail that shows the original document with all the artboards. In Windows all files will show the same thumbnail. As a fix I now use a little script that opens each file, makes a change, undoes the change and saves the file again. This will generate the correct thumbnail.

Why can't Illustrator just generate the correct thumbnail when you save the file?