Update to this issue – was still happening on multiple client sites.
I’m pretty sure the offending plugin is SEOPress – have seen some reports that Yoast may be causing similar problems. Boils down to the time taken to do the SEO/Content analysis when the post loads in editor, and that causing the featured image panel load to just time-out.
I have now tried this approach (disabling SEOPress plugin, at least when creating/editing posts) and every time the Featured Image panel is present and correct.
Have posted on the plugin forum to see if they have any suggestions to remedy this.