It seems something is happening with the thumbnail image that Kindlegen supposed to produce for the KF8 version. It correctly creates the thumbnail image for the Mobi7 but not for the Mobi8 set.
You can see the files compiled by Kindlegen using the
KindleUnpack script on your MOBI file. Kindlegen supposed to make 2 images for your KF8 version of the book: (1) for the cover and (2) for the thumbnail but it doesn't. I don't know what is preventing your cover image from not correctly being converted since it works on Mobi7 version.