Why is the same line sometimes printed in a separate C++ thread?

Question

C++ / C#

Why was the unit printed twice in the 3rd line?

void foo(){
    for(int i = 0; i < 5; i++){
        cout << "cout " << i << endl;
    }
}
int main() {
    thread t(foo);
    t.detach();
    assert(!t.joinable());
    std::cout << "Hello, World!" << std::endl;
    return 0;
}

output is as follows (one of many options):

Hello, World!cout 
0
cout 1cout 1
cout 2
c

Answer 1

most likely due to the effect of flush when calling < std::endl