I noticed that the last page didn't have the <h6> PG Trailer centered, so I retried EText-No. 611 and choose 'center all <h1..h6>' which, in english, means ensure all header tags <h1> to <h6> are centered, even if already defined as so. It did center in this case.
You will note that the chapter header tags used by this HTML are "hard-coded" as being ALIGN="center"
and do not make use of internal CSS like many newer offerings do.
It's a safe bet to 'check' this GUI option all the time (just set it and forget it!).