James, I think there might have been a caching/timing thing after all because OG images are now displaying. So, my WhatsApp link previews are now working perfectly.
Just so I can mark this issue as solved…
- The issue was that the Minify HTML plugin was stripping out some closing HTML tags, most crucially the
</head>tag, as highlighted by Sonja. - The solution was to add a plugin config option to the config.php file –
'doRemoveOmittedHtmlTags' = false
Thanks, both, for your help.