I’m pretty sure I solved the problem.
In my custom theme’s “event-single.twig” file was the following if statement:
{% if event.get_runtime( 'content_img_url' ) is empty %}
code that displays the featured image
{% endif %}
Once I commented the if statement out like so
{# % if event.get_runtime( 'content_img_url' ) is empty % #}
code that displays the featured image
{# % endif % #}
The featured image displayed correctly.
https://www.newseum.org/event/inside-media-1965-civil-rights-at-50/
The same if statement is in one of the calendar plugin’s “Vortex” theme.
Anyone know what its purpose is?