Thanks to all for pointing out the error on the site. It’s an open bug on GitHub for Automattic’s JetPack.
Here’s the fix from the bug thread:

The following change to the core function get_the_content() eliminates the problem

if ( is_array( $pages ) ) {
    if ( $page > count( $pages ) ) // if the requested page doesn't exist
        $page = count( $pages ); // give them the highest numbered page that DOES exist
} else {
    $page = 0;
}

… which I did from my Nexus 6P via ssh in a Termux session!