In my plugin I define a settings like so : 

I then proceed to have my settings.php file have these contents: 

$form->add(new \IPS\Helpers\Form\TextArea('disable_multiple_quantities_names', \IPS\Settings::i()->disable_multiple_quantities_names, null, array()));

if ($values = $form->values()) {
    $form->saveAsSettings();
    return true;
}

return $form;

 

Going to the settings section of the plugin, I get this as expected (Disregard the plugin name, its for testing) : 

 

Putting in a value and pressing save, I get this: 

 

 

 

 

Edit : 

Creating a new plugin and redoing everything fixed this, something must have been corrupted.

Just to flag the 'best answer', the OP recreated his plugin as something apparently got corrupted.