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The documentation for multiprocessing.exitcode says:
"""
 The child’s exit code. This will be None if the process has not yet terminated. A negative value -N indicates that the child was terminated by signal N.
"""
This is true for the "fork" method, but not "forkserver" where a child terminated by a signal will get an exitcode of 255. This is because forkserver relies on the child writing its own exit code in a pipe, which obviously doesn't work if it was killed (255 is simply a fallback value).
See forkserver's Popen.poll():
 def poll(self, flag=os.WNOHANG):
 if self.returncode is None:
 from multiprocessing.connection import wait
 timeout = 0 if flag == os.WNOHANG else None
 if not wait([self.sentinel], timeout):
 return None
 try:
 self.returncode = forkserver.read_unsigned(self.sentinel)
 except (OSError, EOFError):
 # The process ended abnormally perhaps because of a signal
 self.returncode = 255
 return self.returncode