SearchWP's Term Highlight Extension does not automatically highlight terms in Supplemental Search Engine results templates, but it is easy to implement highlight functionality.

gistfile1.php

<?php

// this snippet is based on SearchWP's sample Supplemental Search Engine results template found here:
// https://searchwp.com/docs/configuration/#supplemental


// **************************** NOTE ****************************
// this snippet is just a portion of your search results template
// **************************** NOTE ****************************

// these are the search terms
$query = isset( $_REQUEST['swpquery'] ) ? sanitize_text_field( $_REQUEST['swpquery'] ) : '';

$highlighter = false;
if( class_exists( 'SearchWP_Term_Highlight' ) ) {
    $highlighter = new SearchWP_Term_Highlight();
}

?>

<?php foreach ( $posts as $post ): setup_postdata( $post ); ?>
    <div class="post">
        <h2>
            <a href="<?php echo get_permalink(); ?>">
            <?php
                // highlight the title
                $title = get_the_title();
                if( $highlighter ) {
                    $title = $highlighter->apply_highlight( $title, $query );
                }
                echo wp_kses_post( $title );
            ?>
            </a>
        </h2>
        <?php 
            // output an excerpt
            $excerpt = get_the_excerpt();
            if( $highlighter ) {
                $excerpt = $highlighter->apply_highlight( $excerpt, $query );
            }
            echo wp_kses_post( $excerpt );
        ?>
        <div class="custom-field-content">
            <?php 
                // output highlighted content from a Custom Field
                $custom_field = get_post_meta( get_the_ID(), 'my_custom_field_key', true );
                if( $highlighter ) {
                    $custom_field = $highlighter->apply_highlight( $custom_field, $query );
                }
                echo wp_kses_post( $custom_field );
            ?>
        </div>
    </div>
<?php endforeach; ?>