I just compiled a book and it has no book cover image on the icon or when added to Apple Books.
Mac 15.2 (24C101) - Pandoc pandoc 3.6.1 - iMac M3.

So I then proceeded to compile a book I had compiled under a previous version of Pandoc. This too had no cover image icon on Mac finder or in apple books.

Some images might make it clearer:
In Finder:

In Apple Books:

Same book, same compile system but different results?

The last time the original books was compiled was August 6th 2024, so I guess it would have been pandoc 3.3 2024-07-28 via homebrew.

I had a look at the Release notes for later versions of pandoc and checked if anything had changed to the epub image and could only find:
"pandoc 3.6 (2024-12-07)
EPUB writer: Use standardized filename for cover image instead of the original name (#10404). This avoids problems with e.g. filenames containing spaces."

Anyone any ideas?