Thumbnails no maintaining reduced size

I have numerous books set to 'facing page' with added thumbnails. I also set them for 'Fit Spread" so I can utilize the monitor size appropriately. However, with the page pulled out to occupy the full width and length, there is a slight amount of space at the bottom which is easily adjusted.

The problem come if I adjust the thumbnail section smaller and save it. I works great until I close that document and later reopen it that the thumbnail section reverts to its default size. And, this has already happened every time I open a new book, they are all from pages09, add thumbnails back, adjust the page and thumbnail as noted the thumbnail reverted to default every time they are reopened.

Anyone with a workaround until it is fixed?

Just as an afterthought, Why does the conversion from 09 to Big Sur drop the previously added thumbnail but retain facing page?