Comments & Replies no longer visible since upgrade to 5.x

I’m in the process of moving my site to a new host and while running a functionality check in staging I noticed that visitor comments are no longer showing up in my theme – just a HTML comment that looks like a placeholder. Did something change in the way that comments are handled in the last few versions?

<div id="comments" class="comments-area">
	<h2 id="comments-title">
		<i class="icon-comments"></i>2 comments for “<span>Garmin 3D Speed & Distance Test</span>”
	</h2>
	<ol class="commentlist">
		<ul class="children">
			<!-- #comment-## -->
		</ul>
		<!-- .children -->
		<!-- #comment-## -->
	</ol>
	<!-- .commentlist -->
	<p class="nocomments">Comments are closed.</p>
</div>

To eliminate my many customisations I span up a brand new instance, updated it to 5.5.3, and installed the freshly-downloaded theme. Same result.

To be clear, this isn’t a case of a site / post configuration preventing comments, it’s a case of existing comments not being rendered due to some kind of recent change. I’d be very grateful for any insights.

EDIT: I just created another fresh WP instance, installed the same theme, but this time didn’t update it, leaving it instead at 5.4.4. Comments do show up in this older version, so it’s something weird between 5.4.4 and 5.5.3.

The page I need help with: [log in to see the link]