Default Values of Multiselect Input in Widget Options Using widget.xml

The method getAttributeText looks like this:

public function getAttributeText($attributeCode)
{
    return $this->getResource()
        ->getAttribute($attributeCode)
            ->getSource()
                ->getOptionText($this->getData($attributeCode));
}

Notice the last 2 lines.
They mean that the method calls getOptionText from the source model class.
I see that your attribute has a custom source model npm_recurrex/package_source. Make sure that model has the method getOptionText or it's parent class has it and returns what your need.

[EDIT]
Your getOptionText is wrong.
It would work if the attribute would be of type select.
But it doesn't work for multiselects.
Here is a scenario. Actually I'm using your scenario.
$_product->getData('package') returns 1,2.
And I assume your options are something like this:

$options[] => array('value' => 1, 'label'=>'Option 1');
$options[] => array('value' => 2, 'label'=>'Option 2');
$options[] => array('value' => 3, 'label'=>'Option 3');
$options[] => array('value' => 4, 'label'=>'Option 4'); 

In this case, your method receives as parameter 1,2 and you check if there is an element in $options with value = 1,2. and there isn't.
Try to make your method look like this:

public function getOptionText($value)
{
    $options = $this->getAllOptions(false);
    $optionsByValue = array();
    foreach ($options as $item) {
        $optionsByValue[$item['value']] = $item['label'];
    }

    $values = explode(',', $value);
    $returnValue = array();
    foreach ($values as $_value) {
         if (isset($optionsByValue[$_value])) {
             $returnValue[] = $optionsByValue[$_value];
         }
    }
    return implode(', ', $returnValue);
}

Untested code so look out for typos.

If default values are not working for YesNo (I don't have time to dig) then you could always cheese the system for the time being and try to find a cleaner solution:

<parameter name="chart_layout" xsi:type="select" visible="true">
    <label translate="true">Full Width</label>
    <options>
        <option name="yes" value="1" selected="true">
            <label translate="true">Yes</label>
        </option>
        <option name="no" value="0">
            <label translate="true">No</label>
        </option>
    </options>
</parameter>