Further to the above, is anyone aware of any limiations on Stack References? For example, can you have a Stack using Pulumi.Aws v 4.30.0 and another Stack that has a Stack Reference to it that uses a different Pulumi.Aws version? I presume this is a supported setup?

I'm not aware of why a StackReference would care about what plugins the referenced stack is using - since my understanding was that a StackReference would just be parsing the state of the referenced stack in order to retrieve any outputs. Perhaps my understanding is flawed. Are you certain that it is the StackReference that is causing your hanging?

I have 2 stacks using Pulumi.Aws v4.5.1: a network stack and an application stack. The application stack has a stack reference to the network stack, but not a project referrence. The 2 stacks share no dependencies. I can upgrade the network stack to v4.30.0, but if I then try to upgrade the application stack it hangs. I can then revert the network stack back to 4.5.1 and pulumi up on the application stack is then OK. I can then try to upgrade the application stack and leave the network stack on v4.5.1. The result: pulumi up on the application stack hangs again.