Don’t you use gravatar in your website?
This is a new feature, users can navigate to other messages from there. But there should show recipients avatar.

If you do not like this feature there is an easy way to show like previous version. Let me know if you want that.

Add following code in your theme’s (child theme’s if you are using) functions.php

add_filter( 'fep_locate_template', function( $template, $template_names ) {
	if ( in_array( 'view-message.php', $template_names ) ) {
		$template = fep_locate_template( 'view-message-legacy.php' );
	}
	return $template;
}, 10, 2 );