printf anomaly after "fork()"

OS: Linux, Language: pure C

I'm moving forward in learning C programming in general, and C programming under UNIX in a special case.

I detected a strange (for me) behaviour of the printf() function after using a fork() call.

Code

#include <stdio.h>
#include <system.h>

int main()
{
    int pid;
    printf( "Hello, my pid is %d", getpid() );

    pid = fork();
    if( pid == 0 )
    {
            printf( "\nI was forked! :D" );
            sleep( 3 );
    }
    else
    {
            waitpid( pid, NULL, 0 );
            printf( "\n%d was forked!", pid );
    }
    return 0;
}

Output

Hello, my pid is 1111
I was forked! :DHello, my pid is 1111
2222 was forked!

Why did the second "Hello" string occur in the child's output?

Yes, it is exactly what the parent printed when it started, with the parent's pid.

But! If we place a \n character at the end of each string we get the expected output:

#include <stdio.h>
#include <system.h>

int main()
{
    int pid;
    printf( "Hello, my pid is %d\n", getpid() ); // SIC!!

    pid = fork();
    if( pid == 0 )
    {
            printf( "I was forked! :D" ); // removed the '\n', no matter
            sleep( 3 );
    }
    else
    {
            waitpid( pid, NULL, 0 );
            printf( "\n%d was forked!", pid );
    }
    return 0;
}

Output:

Hello, my pid is 1111
I was forked! :D
2222 was forked!

Why does it happen? Is it correct behaviour, or is it a bug?