@michael-n-cooper either Master hasn't been rolled out so the published version is out of sync or the generator is overwriting ids.

In Master (line 36 of https://github.com/w3c/wcag/edit/master/understanding/21/status-messages.html):
<h3 id="status-examples">Status Message Examples</h3>

In the live site:

<section class="example" id="status-message-examples">
          <h3>Status Message Examples</h3>

i believe (though my xslt is a bit rusty) that it's the generator (probably base.xslt) explicitly creating ids based on the heading text and overriding the id attribute already in the markup. cleanest fix would likely be to modify the xslt if possible NOT to generate custom ids if the attribute is already present. /cc @michael-n-cooper

[second edit] the reason for this seems even more complex ... removing my incorrect guess

(and yes, funnily enough i came here because i was going to file an issue about those relative links in 4.1.3 understanding...)

even odder...in master, the understanding document has

<section class="example">
<h3 id="status-examples">Status Message Examples</h3>

(with the id on the <h3>) but in the ouput https://www.w3.org/WAI/WCAG21/Understanding/status-messages.html it has

<section class="example" id="status-message-examples">
<h3>Status Message Examples</h3>

(with the id stripped from the <h3> and the new id added to the <section>)

[edit] and after adding correct code fences to @awkawk's previous comment, i see that he saw the same oddness...apologies for rehashing the same thing