dispatch_async works as expected

dispatchTest.m

// this behaves as expected

- (void)exampleOne {
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:10];
        NSLog(@"10s --- %@", [NSThread currentThread]);
    });

    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:5];
        NSLog(@"5s --- %@", [NSThread currentThread]);
    });
    
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:5];
        NSLog(@"5s --- %@", [NSThread currentThread]);
    });
    
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:5];
        NSLog(@"5s --- %@", [NSThread currentThread]);
    });
}

// so does this

- (void)exampleTwo {
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:10];
        NSLog(@"10s --- %@", [NSThread currentThread]);
    });
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        dispatch_async(queue, ^{
            [NSThread sleepForTimeInterval:5];
            NSLog(@"5s --- %@", [NSThread currentThread]);
        });
        
        dispatch_async(queue, ^{
            [NSThread sleepForTimeInterval:5];
            NSLog(@"5s --- %@", [NSThread currentThread]);
        });
        
        dispatch_async(queue, ^{
            [NSThread sleepForTimeInterval:5];
            NSLog(@"5s --- %@", [NSThread currentThread]);
        });
    });
}

Pursuant to this Stack Overflow question, dispatch_async works as expected (as illustrated by the above); only dispatch_after manifests the curious behavior you see in that question.