Thank you for your response. I have been looking into it further and was about to update/edit my original post.
It seems that the addition of libTidy had caused the issue, and that forcing it to not use Tidy fixed our issue.
The libTidy release in question is "2017/11/25". After attempting to reproduce the issue elsewhere, it seems that libTidy "2017/03/01" works just fine and that our production libTidy is "too new" or has another issue we aren't seeing.
Right now, forcing the "_load_html()" function to take the non-Tidy branch fixes our issue.