Add an #ajax callback to a FieldWidget and triggering it it will lose the properties set in __construct during submit

Problem/Motivation

When you add an #ajax callback to a FieldWidget it will lose the properties that you set in __construct during submit.

E.g.

public function __construct() {
$this->testProperty = 'test property';
}

$widget['widget']['#ajax'] = [
      'callback' => array($this, 'changeSelectedRoles'),
      'event' => 'change',
      'progress' => array(
        'type' => 'throbber',
        'message' => t('Change selected roles...'),
      ),
    ];

When you submit the form without triggering the ajax callback it will show $this->testProperty as 'test property''.
When you submit the form with triggering the ajax callback it will show $this->testProperty as NULL.