The issue I reported above may be due to the structure of my site, but I fixed it by changing this:
(on line 256 of yoast-breadcrumbs.php)
$output .= bold_or_not(get_the_title());
to
$output .= bold_or_not(get_the_title($wp_query->post->ID));
I didn’t dig into WP too far to find out what get_the_title provides when you don’t give it an ID, but the issue seemed to lie with that. I hope this helps someone.