Regarding parallel POSIX threads

I want to create two threads, which are supposed to look like this:

P1:

while(1) {
    printf("1");
    printf("2");
    printf("3");
    printf("4");
}
return NULL;

P2:

while(1) {
    printf("5");
    printf("6");
    printf("7");
    printf("8");
}
return NULL;

According to my knowledge of parallel threads it won't print 12345678 but a completely random variation of the numbers due to lack of synchronization.

However when I try to replicate it in real code it keeps printing 1234 a few times, then switches to 5678, prints it a few times and goes back to 1234.

Is my understanding of threads wrong or is my code not equivalent to the problem?

void *print1(void *arg) {
    while(1) {
        printf("1");
        printf("2");
        printf("3");
        printf("4\n");
    }
    return NULL;
}

void *print2(void *arg) {
    while(1){
        printf("5");
        printf("6");
        printf("7");
        printf("8\n");
    }
    return NULL;
}


int main() {
    pthread_t tid1, tid2;
    pthread_create(&tid1, NULL, print1, NULL);
    pthread_create(&tid2, NULL, print2, NULL);
    pthread_join(tid1, NULL);
    pthread_join(tid2, NULL);
    return 0;

}