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!