Magento2 – How to Get Product Attribute in Cart

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:
Get specific attribute for each product on catalog page

Output custom text attribute in catalog list.phtml (Magento 2)