/usr/bin/id always reports default primary group as effective, even if it isn't

Issue

• Running a suid binary that:
  • calls setgid(2) to set the group ID of the calling process
  • calls setegid(2) to set the effective group ID of the calling process
  • call setgroups(2) to clear the supplementary group list
• From this process run /usr/bin/id -G or groups
• id(1) or groups(1) will falsely reports the primary group from /etc/passwd as effective

# id -a user1
uid=501(use1r) gid=501(user1) groups=501(user1),1010(group1010)
# su - user1
$ /path/to/suidbinary         # This binary calls execve to run id -G after calling setgid()/setegid()
1010 501