CiviCRM 5.39 for WordPress creates an issue for some sites which is solved using phdccltd/civicrm-539-wp-content-fix.
For sites that don't have this issue, this plugin forces it to appear by calling wp_trim_excerpt() before the main output is returned for display.
Follow these instructions to download this plugin's zip file and then upload via Plugins -> Add new on your WordPress site.