After more digging, I found that the following happens:

popen_fork.py -> _launch(self, process_obj) -> self.pid = os.fork()

When I let process (both child and parent) print resulting pid, on freezing I can see:
a) 50-times pid > 0
b) 49-times pid == 0

That means the parent is aware of 50 children, while only 49 of them get to the next line. Not sure if the one remaining process crashes on segfault, but parent apparently hangs later in os.waitpid() on this valid pid of the missing child.