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> The value of exit returned to the parent should be status & 0377.
Apparently, this is not so on Windows. See msg241903 in #24045.
POSIX defines [1] exit to return status & 0377, but that does not mean that sys.exit(256) must return 0 without a warning. It is very unlikely that someone would intentionally use code=256 to signify success. It is much more likely that sys.exit(256) is a result of a programming error.
I believe a better behavior for sys.exit() would be to truncate the code values to 8-bit range so that non-zero status would always be returned as non-zero, but possibly different value.
[1] http://pubs.opengroup.org/onlinepubs/009695399/functions/exit.html