Entity 'changed' column doesn't change upon save
Okey I found the final reason: 

ChangedItem.php

  $original = $entity-&gt;original;
      $langcode = $entity-&gt;language()-&gt;getId();
      if (!$entity-&gt;isNew() &amp;&amp; $original-&gt;hasTranslation($langcode)) {
        $original_value = $original-&gt;getTranslation($langcode)-&gt;get($this-&gt;getFieldDefinition()-&gt;getName())-&gt;value;
        if ($this-&gt;value == $original_value &amp;&amp; $entity-&gt;hasTranslationChanges()) {
         $this-&gt;value = REQUEST_TIME;
        }
      }


The conclusion:

In Drupal 8 if you save the same entity TWICE within 1 second (for whatever reason), you won't able to save your own changed time BUT it will be the REQUEST_TIME instead... wtf? ...