How to preserve element type validation upon adding custom element validation? I have ConfigFormBase. 
It has a form element with '#type' =&gt; 'entity_autocomplete'.

When I add my custom element validation by '#element_validate' =&gt; [[get_class($this), 'validateReferencedItemExistence']] the original element validation which comes by the element type (Drupal\Core\Entity\Element\EntityAutocomplete::validateEntityAutocomplete) won't be added anymore.

After spending hours in the jungle of the FormBuilder class code I finally found the corresponding lines:

   if (isset($element['#type']) &amp;&amp; empty($element['#defaults_loaded']) &amp;&amp; ($info = $this-&gt;elementInfo-&gt;getInfo($element['#type']))) {
      // Overlay $info onto $element, retaining preexisting keys in $element.
      $element += $info;
      $element['#defaults_loaded'] = TRUE;
    }


I find this kinda bad approach, that you cannot extend the list of the validation items. What is the reason behind it?

How could I make a workaround / solve this issue?

(Just by calling Drupal\Core\Entity\Element\EntityAutocomplete::validateEntityAutocomplete in my custom validation I would create an unnecessary depedency in my code. (Just think about if they change their way of validation with a different function))
You need add the definitions defined in the getInfo() method of that type explicitly yourself, there is currently no other way to do that.