Probably yet-another-canonical-link issue [0] [1]. This has been happening regularly recently (and I realized just now is why couple of my recent submissions got borked). This is in the HTML source:

  <link data-react-helmet="true" rel="canonical" href="https://understandlegacycode.com/"/>

[0] https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...

[1] https://en.wikipedia.org/wiki/Canonical_link_element

Correct. Fixed above now.

I assume the submitter can re-edit the link and fix it, if they notice the problem, so perhaps a warning message to double-check after submission can be included?

The submitter cannot, only the title (which I've used when HN changed it on me, particularly with acronyms).