No, there's been no change in anything relevant. The BBCode works in isolation too.
$configurator = new s9e\TextFormatter\Configurator;
$configurator->BBCodes->addCustom(
'[size={CHOICE=large,small,xx-small,x-small,medium,x-large,xx-large}]{TEXT}[/size]',
'<span style="font-size:{CHOICE}">{TEXT}</span>'
);
extract($configurator->finalize());
$text = '[size=small]..[/size]';
$xml = $parser->parse($text);
$html = $renderer->render($xml);
die("$html\n");
<span style="font-size:small">..</span>