Smarty::setExtensions() killing registered filters

Instructions to add output filters are:

$smarty->registerFilter("output", "protect_email");

Instructions to add custom extensions (overriding the defaults) are:

$smarty->setExtensions([
    new Smarty\Extension\CoreExtension(),
    new MyCustomExtension(),
    new Smarty\Extension\DefaultExtension(),
]);

Unfortunately the latter kills $smarty->BCPluginsAdapter thus disabling any filters. I solved the problem by using:

$extensions = $smarty->getExtensions();
array_splice($extensions, 1, 0, [new MyCustomExtension]);
$this->setExtensions($extensions);

I don't like the style, but at least it is working. But something must be done here, either in the API, or at least the documentation should be updated.

Start with Smarty::setExtensions(), getExtensions(), and BCPluginsAdapter, then reproduce the reported interaction between custom extensions and registered output filters. Done means the filter remains usable after extensions are configured, or the API documentation clearly explains the required behavior and workaround.