I'm trying to grab a product attribute in the cart template. But I just does not work! The attribute is set to Visible on Catalog Pages on Storefront = yes and Used in Product Listing = yes

I use a custom module to override the following template:

    /vendor/magento/module-checkout/view/frontend/templates/cart/item/default.phtml

 I have the product:

    $_item = $block->getItem();
    $product = $_item->getProduct();

Normally I would expect a custom attribute (of type text-area in this case) with an attribute code of 'sample_attr' to be available with one of these:

    $product->getData('sample_attr');
or

    $product->getSampleAttr();

Some attributes are available using this (sku, weight, price ..), but for a custom attribute neither works. 
The included helper (\Magento\Msrp\Helper\Data) also has no attribute method.

I can get all attribute codes using:

       $attributes = $product->getAttributes();
       foreach($attributes as $a)
       {
         var_dump($a->getName());
       }


I've also tried:

        $attribute = $product->getResource()->getAttribute('sample_attr');
        var_dump($attribute->getFrontend()->getValue($product));

but this returns null.

So how do I get the value of the attribute?

I've tried these:
http://magento.stackexchange.com/questions/4330/get-specific-attribute-for-each-product-on-catalog-page?rq=1

http://magento.stackexchange.com/questions/110402/output-custom-text-attribute-in-catalog-list-phtml-magento-2