Retested it with a freshly installed 3.6.4 version. Used the following code to test:

import sys
import multiprocessing


def foo():
    return 'bar'


if __name__ == '__main__':
    proc = multiprocessing.Process(target=foo)
    proc.start()
    proc.join()
    with open('process_exit_code.txt', 'w') as f:
        f.write(sys.version)
        f.write('\nprocess exit code: ')
        f.write(str(proc.exitcode))

It is very important to run the script with pythonw, not just with python. This is the content of the resulting process_exit_code.txt file on my machine:
3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)]
process exit code: 1

As it can be seen the problem was not fixed. The process exit code should be 0. By default the new multiprocessing process created uses the same interpreter as the creator process, so it uses pythonw too.