title: Magento 2 cart price rule and tax issue
body: <p><strong>Specs</strong>: Magento 2.1.7</p>
  
  <p>I got an issue and could not find anything similar to my case on the google. </p>
  
  <p>I have a tax price rule that makes free shipping when subtotal is above 500 euros.
  Rule is working fine, free shipping is being applied without problems. 
  Problem comes with products that have FPT (Fixed product tax) applied.</p>
  
  <p>When free shipping is applied, tax amount from the totals turns into a nice round <strong>zero</strong>. Tax amount reduced to the <strong>zero</strong>.</p>
  
  <p>Apparently this isn't happening with products that are using tax classes.
  Shipping becomes free and taxes looks fine.</p>
  
  <p>I'm out of the ideas, going to dive into the code, but I'm not sure if I will find the problem.
  Thanks for any help.</p>