Unexpected behavior in the file access control, resulted from the mixture of specifications about the supplementary group list

Issue

• Following two unexpected behaviors occur if certain conditions are met:
  1) Fail to open a file though the third group of the permission bits of the file are set (it is expected that the file is accessible).
  2) Succeed to open a file though the third group of the permission bits of the file are not set (it is expected that the file is not accessible).

The conditions are as follows:
1. Start a process with "root" euid and an egid which is the same as the group ID of the file.
2. setgid() to a group which is different from the group ID of the file.
3. setuid() to a user which is different from the owner ID of the file.

Sample Program:
# cat /tmp/sample.c
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>

int main(int argc, char *argv[])
{
        setgid(1);
        setuid(1);
        int fd = open(argv[1], O_RDONLY);
        if (fd < 0)
                perror("open");
        return 0;
}
# cd /tmp
# make sample
cc     sample.c   -o sample
# id
uid=0(root) gid=0(root) groups=0(root),10(wheel)

Execution Sample:
* In order to confirm 1), execute the above program by the following file: 
# ls -la /tmp/file
 -rwx---rwx 1 root root 0 Mar  5 11:07 file
        ^^^
# ./sample /tmp/file
 open: Permission denied (You cannot open the file)

* In order to confirm 2), execute the above program by the following file: 
# ls -l /tmp/file
-rwxrwx--- 1 root root 0 Mar 28 17:35 /tmp/file
       ^^^
# ./sample /tmp/file
#  (You can open the file)