.epub file blocked despite ALLOW_UNFILTERED_UPLOADS set to true

You probably need to allow the mime types for them to be allowed to upload.

https://www.robertwent.com/blog/adding-custom-mime-types-for-wordpress-uploads/

//The following goes in a themes functions file or a custom hooks plugin

function so_387865_custom_upload_mimes ( $existing_mimes ) {
    $existing_mimes['epub'] = 'application/epub+zip';
    $existing_mimes['mobi'] = 'application/x-mobipocket-ebook';
 
    return $existing_mimes;
}
 
add_filter('upload_mimes', 'so_387865_custom_upload_mimes');