I’m writing a program that requires admin privileges (an updater). At some point, we need to start another program and let the first one end. So the parent process makes a fork, detaches from its child and ends execution.
As root (using “su” for exemple), everything works as expected.
As a regular user, nothing special happens (except of course for writing files as root).
But, using sudo, as soon as the parent process ends its execution, its child dies as well.

I wrote a minimal program to debug this issue:

int main()
{
    std::cout << "start: " << getuid() << std::endl;
    if (auto pid = fork() == 0)
    {
        setpgid(0, 0);
        std::cout << "fork has detached" << std::endl;
        sleep(1);
        std::ofstream test("/root/test.txt");
        std::cout << "fork: " << getuid() << " " << test.is_open() << std::endl;
    }
    else
    {
        setpgid(pid, 0);
        std::cout << "parent: " << getuid() << std::endl;
    }
    return 0;
}

I expect as output:

start: 0
parent: 0
fork has detached
fork: 0 1

But with sudo I only get:

start: 0
parent: 0
fork has detached

If I add a wait() call in the parent, the child works as expected with sudo. So the child dies with its parent. The debug tools I tried only showed traces for the parent, how can I track the detached child as well?

Why does using sudo makes my program behave this way?
I can work around this issue for my project, but I’m interested in the explanation of this strange behaviour.