Using the intrinsic function Ref on a Stack created within another Stack only gives you the Id of the referenced Stack. If you want to get the StackName which is generated automatically you have to do a combination of the intrinsic function Split and Select as follows:
!Select [1, !Split ["/", !Ref MyStack]]
This works since the Stack Id is structured as follows:
The statement splits the Stack Id by deleimiter “/” resulting a list which contains the StackName on index 1 which we then can select using the Select function.