By reading the GA docs I came to the conclusion that this is a limitation of the measurement protocol and therefore GA itself.

You can find the relative document here: https://developers.google.com/analytics/devguides/collection/protocol/v1/parameters

The description of the "Product Action List" parameter says that the product list of a product impression object only gets attributed to the "detail" and "click" actions. 

In other words, if you send the "Product Action List" parameter alongside a "Product Action" parameter whose value is anything else except "detail" or "click", then it's highly likely that the issue you described will appear in the "Product List Performance" report.

Using the example illustrated here:
https://developers.google.com/analytics/devguides/collection/analyticsjs/enhanced-ecommerce

This command won't work in our case:
ga('ec:setAction', 'add', {list: 'Search Results'});

But this will:
ga('ec:setAction', 'click', {list: 'Search Results'});

The problem occurs when the user inserts the product to his cart ("add" action) without interacting ("click" action) or viewing the details of ("detail" action) of it. 

One solution would be to send two events with similar GA ecommerce parameters when a user adds the product directly to his cart. The prior would be an interactive and the latter a non interactive event or vice versa.

Here's an example:

I'm logged into a ecommerce website, just landed on a web page and viewed all of the page's products (pageview with relevant enhanced ecommerce impression data is already sent). I now add a product with the product list "TVs" to my cart. At this moment two GA events should be fired:

1) One with ga('ec:setAction', 'click', {list: 'TVs'});

2) The other with ga('ec:setAction', 'add', {list: 'TVs'});

I managed to get this working by adding the action field attribute to the dataLayer call.  Like this:

dataLayer.push({
  'event': 'addToCart',
  'ecommerce': {
    'currencyCode': 'EUR',
    'add': {  
      'actionField': {'list': 'Search Results'},                         
      'products': [{
        'name': 'Triblend Android T-Shirt',
        'id': '12345',
        'price': '15.25',
        'brand': 'Google',
        'category': 'Apparel',
        'variant': 'Gray',
        'quantity': 1,
        'list': 'Search Results'
       }]
    }
  }
});

That worked for me.

I had the same problem and was stuck for hours. 

My issue was that I had the checkout process on a different subdomain than where the product impressions was listed and clicked on. 
By simply adding a field on the GA page view tag i GTM, "cookieDomain" = "auto", all started working. 
http://www.lunametrics.com/blog/2015/06/16/cross-domain-tracking-with-google-tag-manager/

The only places I needed to add information about the list, was:
- product impressions list. "'list' : 'Search result'"
- product click (from impressions list). 'actionField': {'list': 'Search Results'},

I did not add it for product detail, or anything in the checkout process (add to cart, checkout, purchase).

For other external domains in the checkout process I guess you will need cross domain tracking.

We're having the same problem.

We've tried all of the suggestions above, but are still seeing a large number of (not set)'s showing around (45%) for the product checkouts list.

 Issue 750  has been merged into this issue.

I have the same problem, 

Same here. Currently experiencing a very high volume of revenue associated to product list name set as "(not set)". Looking forward to see if GA tech team will be fixing this soon.

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