为什么libxml_use_internal_errors（true）不尊重LIBXML_NOWARNING？

I am tying to understand how libxml_use_internal_errors(true) works. It does not seem to respect the LIBXML_NOWARNING flag.

Here is an example using PHP 7.2.4.

libxml_use_internal_errors( false );
new SimpleXMLElement(
  '<DataSet xmlns="iminvalid"></DataSet>', 
  LIBXML_NOWARNING );
var_dump(sizeof(libxml_get_errors()));
// int(0)

As expected, there are no warnings spit out by these statements. The namespace warning is hidden because of LIBXML_NOWARNING.

However, this:

libxml_use_internal_errors( true );
new SimpleXMLElement(
  '<?xml version="1.0" encoding="utf-8"?><DataSet xmlns="iminvalid"></DataSet>', 
  LIBXML_NOWARNING );
var_dump(sizeof(libxml_get_errors()));

// int(1)
// xmlns: URI iminvalid is not absolute

Unexpectedly returns the invalid uri warning. How is it possible to libxml_use_internal_errors( true ) and hide xml parsing warnings?