I think this is a bug but want to invite comments before reporting it.
The book was "Arsène Lupin, super-sleuth" by Maurice Leblanc,
https://www.gutenberg.org/ebooks/74269.
When viewed online at
https://www.gutenberg.org/cache/epub...69-images.html the first chapter's first page had no visible footnotes (I don't know for sure that footnotes are displayed when viewing online).
After downloading the "EPUB3 (E-readers incl. Send-to-Kindle)" version, importing to Calibre 7.20.0 and opening the book, the first chapter's first page had footnote number 12 after "make a careful examination of these two persons". No footnote footer is displayed and I was not able to open the footnote (I don't read books in Calibre so am not familiar with footnote usage). The last chapter's last page had footnote 320 after "The child, corrupted too early, could not rise from the depths".
After using Calibre 7.20.0 to send the book to a Kobo Aura H2O and opening the book, the same footnotes were visible. When opened they showed the content of the page they were on.
One unusual feature of the book which may have triggered the bug is a colophon listing 50 corrections with links to the corrected text which appears with a faint underline in the text