I think I've found the answer: the thread is sometimes already stopped by the time the child is forked, so it doesn't appear in _enumerate() anymore (it left the _active dict). Therefore its locks are not reset in _after_fork().

Oh, I also get the following sporadic failure which is triggered by slight change in semantics with Thread.join(timeout) :-)

======================================================================
FAIL: test_various_ops (test.test_threading.ThreadTests)
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Traceback (most recent call last):
  File "/home/antoine/cpython/default/Lib/test/test_threading.py", line 113, in test_various_ops
    self.assertTrue(not t.is_alive())
AssertionError: False is not true