It worked when building the site with v0.59.1 and earlier, which use the Blackfriday markdown processor.
Hugo v0.60.0 was released in November 2019, replacing the Blackfriday markdown processor with Goldmark. By default, Goldmark does not trust HTML within your markdown, and replaces each instance with <!-- raw HTML omitted -->.
Open the problem page in your browser, view source, and search for “raw HTML omitted”.
If you trust the authors of the markdown, place this in your site configuration:
[markup.goldmark.renderer]
unsafe = true
https://gohugo.io/getting-started/configuration-markup#goldmark
unsafe
By default, Goldmark does not render raw HTMLs and potentially dangerous links. If you have lots of inline HTML and/or JavaScript, you may need to turn this on.
You can audit your site prior to deployment to find problems like this.