Bug in JForm->load() implementation

sample_content_plugin.php

# Lets consider a content plugin with the following implementation:
# Basically, its intention is to add/replace elements from a form,
# Joomla 'Global Configuration' form in this case

<?php

    public function onContentPrepareForm($form, $data)
    {
        // The name of the form we want to process
        $formName = 'com_config.application';

        // The xml definition of our additions/replacements
        $fileName = __DIR__.'/forms/'.$formName.'.xml';

        // Only process com_config application forms and only process it if the override file exists
        if ($form->getName() == $formName && is_file($fileName))
        {
            // Attempt to load the XML file as a simpleXml object
            $gaeXml = simplexml_load_file($fileName);

            // Replace XML nodes[aka Joomla Form fields] in this form with ones that have been defined in our replacement file
            $form->load($gaeXml);
        }
    }

?>

# Original definition of a form field from com_config.application

<field
	name="mailer"
	type="list"
	default="mail"
	label="COM_CONFIG_FIELD_MAIL_MAILER_LABEL"
	description="COM_CONFIG_FIELD_MAIL_MAILER_DESC"
	required="true"
	filter="word">
	<option value="mail">COM_CONFIG_FIELD_VALUE_PHP_MAIL</option>
	<option value="sendmail">COM_CONFIG_FIELD_VALUE_SENDMAIL</option>
	<option value="smtp">COM_CONFIG_FIELD_VALUE_SMTP</option>
</field>

# Field definition we want to set instead and also the expected result (added 'gae' option and changed 'default' attribute)

<field
	name="mailer"
	type="list"
	default="gae"
	label="COM_CONFIG_FIELD_MAIL_MAILER_LABEL"
	description="COM_CONFIG_FIELD_MAIL_MAILER_DESC"
	required="true"
	filter="word">
	<option value="gae">COM_CONFIG_FIELD_VALUE_GAE_MAIL</option>
	<option value="mail">COM_CONFIG_FIELD_VALUE_PHP_MAIL</option>
	<option value="sendmail">COM_CONFIG_FIELD_VALUE_SENDMAIL</option>
	<option value="smtp">COM_CONFIG_FIELD_VALUE_SMTP</option>
</field>

# Actual result is as follows (the descendants options are not copied over):

<field
	name="mailer"
	type="list"
	default="gae"
	label="COM_CONFIG_FIELD_MAIL_MAILER_LABEL"
	description="COM_CONFIG_FIELD_MAIL_MAILER_DESC"
	required="true"
	filter="word">
</field>