Dispatch_get_specific sees only the top queue

main.m

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        dispatch_queue_t qa = dispatch_queue_create("a", DISPATCH_QUEUE_SERIAL);
        dispatch_queue_t qb = dispatch_queue_create("b", DISPATCH_QUEUE_SERIAL);
        
        static void* key = &key;
        
        dispatch_queue_set_specific(qa, key, (void*)1, NULL);
        
        NSLog(@"no queue. key = %p", dispatch_get_specific(key));
        
        dispatch_sync(qa, ^(){
            NSLog(@"queue a. key = %p", dispatch_get_specific(key));
        });
        
        dispatch_sync(qb, ^(){
            NSLog(@"queue b. key = %p", dispatch_get_specific(key));
        });

        dispatch_sync(qa, ^(){
            NSLog(@"queue a. key = %p", dispatch_get_specific(key));
            
            dispatch_sync(qb, ^(){
                NSLog(@"queue b on a. key = %p", dispatch_get_specific(key));
            });
        });

    }
    return 0;
}