Taint mode in PHP doesn't work on object that method is called on

[Sjord]

https://semgrep.dev/s/sjord:domdocument_load

  pattern-sources:
  - patterns:
    - pattern: new DOMDocument(...)
  pattern-sinks:
  - patterns:
    - pattern-inside: $DOMDOCUMENT->load($FILENAME, ...)
    - pattern: $DOMDOCUMENT

<?php
$doc = new DOMDocument();
$doc->load('file.xml');  // I expect this to match, it doesn't
$doc->load($doc);  // I expect this to match, it does
$something->load($doc);  // This doesn't match, as expected.

I try to match calls to DOMDocument::load, with taint mode to track whether $DOMDOCUMENT is the result of a call to new DOMDocument(), in $DOMDOCUMENT->load($FILENAME, ...). However, this doesn't seem to work properly. $doc->load('file.xml') doesn't match, even though $doc should be tainted here.