Hi there,
Divi uses shortcodes to display all content and Reading Time WP removes shortcodes from posts as they often are used for functionality that would falsely add to the reading time.
You could modify the plugin yourself to remove the shortcode stripping functionality by changing the following lines in rt-reading-time.php from:
$strippedContent = strip_shortcodes($rtContent);
$stripTagsContent = strip_tags($strippedContent);
to
$stripTagsContent = strip_tags($rtContent);
However, if you make that change you won’t be able to get future updates of the plugin as it would overwrite your changes. You could duplicate the reading time plugin and make a custom version of it with a different name / slug to solve this.
I am working on upgrading the plugin to include support for Gutenberg, but before tackling shortcodes I’m waiting to see how they’ll be implemented in the new editor.