We do not have any control over the Spine Text uploading with your Apple Photos PDF since it was Apple's decision to not include it. This means they include the front and back covers in your PDF, but none of the spine content that you may have created for your book.
For some programs, the spine text you use will be imported, however Apple has made the decision to not include the full dimension cover when you save out your PDF for uploading to another site with Photos.
