That’s really strange. Those hooks are very close to each other within the code – they should work the exact same.
What if you do this?:
add_action( 'wpsp_after_content', 'tu_add_custom_meta_testimonial' );
function tu_add_custom_meta_testimonial() {
global $post;
$meta = get_post_meta( $post->ID, 'testimonial_source', true );
if ( isset( $meta ) && '' !== $meta ) {
echo '<div class="testimonial-source">' . $meta . '</div>';
}
}