`span`s inside `a` tag confuse WordPress's auto-linking

[snarfed]

Originally filed by @edent as snarfed/bridgy#1911 :

I've received this WebMention on my WordPress blog, from Mastodon, via Brid.gy - https://brid.gy/comment/mastodon/@edent@mastodon.social/114053225122484174/114055785692413948

It sends a comment with this HTML (linebreaks added for readability):

<p><span><a href="https://mastodon.social/@Edent" rel="nofollow ugc">@<span>Edent</span></a></span>
 <a href="https://www.imperialviolet.org/2021/08/26/qrencoding.html" rel="nofollow ugc">
   <span>https://www.</span>
   <span>imperialviolet.org/2021/08/26/</span>
   <span>qrencoding.html</span>
 </a> if you want more tricks for densely encoding information in QR codes.</p>

<p><a href="https://www.imperialviolet.org/2021/08/26/qrencoding.html" rel="nofollow ugc">ImperialViolet - Efficient QR codes</a></p>

However, WordPress is mangling it like this:

.

I've disabled all plugins and themes, submitted the raw HTML to a number of plain WordPress blogs, and they all seem to break in the same way.

Is anyone else seeing this, or have I managed to break something 😅