Another problem with posix_spawn() on glibc: it doesn't report errors to the parent process when run under QEMU user-space emulation and Windows Subsystem for Linux. This is because starting with commit [1] (glibc 2.25) posix_spawn()  relies on address space sharing semantics of clone(CLONE_VM) to report errors, but it's not implemented in QEMU and WSL, so clone(CLONE_VM) and vfork() behave like fork(). See also [2], [3].

This can be easily demonstrated:
$ cat test.c
#include <spawn.h>

int main(int argc, char *argv[], char *envp[]) {
    return posix_spawn(0, "non-existing", 0, 0, argv, envp);
}

$ gcc test.c
$ ./a.out
$ echo $?
2
$ aarch64-linux-gnu-gcc -static test.c
$ qemu-aarch64 ./a.out
$ echo $?
0

There is no problem with musl (it doesn't rely on address space sharing).

[1] https://sourceware.org/git/?p=glibc.git;a=commit;h=4b4d4056bb154603f36c6f8845757c1012758158
[2] https://bugs.launchpad.net/qemu/+bug/1673976
[3] https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg04890.html