The redirect is thanks to redirect_canonical()
– we can simply swoop in with a filter and disable it for the front page:
function wpse_184163_disable_canonical_front_page( $redirect ) {
if ( is_page() && $front_page = get_option( 'page_on_front' ) ) {
if ( is_page( $front_page ) )
$redirect = false;
}
return $redirect;
}
add_filter( 'redirect_canonical', 'wpse_184163_disable_canonical_front_page' );
Now you can access the front page at the root and by it’s slug, no redirect.