SpringBoot & nested transaction isolation level override

Spring boot, jdbc.

I feel a bit slippery when it comes to nested transaction..

Say there is MyService that uses MyRepository.

    @Named 
    @Transactional(isolation=Isolation.READ_COMMITTED) // READ_COMMITTED < SERIALIZABLE
    class MyService {
      void do() {
        myRepository.add(entity);
         ...
      }
    }
    @Repository
    class MyRepository {
         @Transactional(propagation=Propagation.REQUIRED, isolation=Isolation.SERIALIZABLE)
         void add(entity) {...}

    }

Say, in this case i want to make sure that MyRepository takes care about the add() method, making sure it set a maximum level lock Isolation.SERIALIZABLE over the table it deals with.

I do not want anyone to override it, since it would lead to concurrency issues.

(same applies to propagation level)

Question:

In case of nested transaction, will transaction's of myService.do() override Isolation level of myRepository.add and maker it less strict ?

What are the precautions when dealing with nested transactions?