This is not a bug in Xcode 11. Xcode 11 includes definitions for new flags that get picked up by Python's build, but only work on a new version of macOS.
I guess this could be seen as a bug in macOS 10.14 (and earlier) because it ignores an unknown flag instead of returning an error. Knowing Apple that's unlikely to get fixed, if it even is a bug (<http://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_spawnattr_getflags.html> says that returning EINVAL is optional for an invalid value of "flags").