test_thread_xslt_parsing_error_log fails with different log length

I've tested both git master and 4.2.2 release; both against libxslt 1.1.30 and 1.1.32 (the only versions available in Gentoo). In all cases, I'm getting the same test failure:

======================================================================
FAIL: test_thread_xslt_parsing_error_log (lxml.tests.test_threading.ThreadingTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib64/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "/tmp/lxml/src/lxml/tests/test_threading.py", line 156, in test_thread_xslt_parsing_error_log
    self.assertEqual(4, len(log))
  File "/usr/lib64/python2.7/unittest/case.py", line 513, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/usr/lib64/python2.7/unittest/case.py", line 506, in _baseAssertEqual
    raise self.failureException(msg)
AssertionError: 4 != 2

The log I'm getting is:

<xslt>:205:0:ERROR:XSLT:ERR_OK: compilation error, element 'foo'
<string>:0:0:ERROR:XSLT:ERR_OK: xsltParseStylesheetTop: unknown foo element

TESTED VERSION: 4.3.0.alpha0
    Python: sys.version_info(major=2, minor=7, micro=15, releaselevel='final', serial=0)
    lxml.etree: (4, 3, 0, -200)
    libxml used: (2, 9, 8)
    libxml compiled: (2, 9, 8)
    libxslt used: (1, 1, 32)
    libxslt compiled: (1, 1, 32)