Point being, that's only if you stop the instance. Since VMs can be migrated live on some vm ecosystems, "changing underneath" is very misleading in this case since it strongly implies a live migration.

If you stop your ec2 instance then you are effectively creating a whole new vm instance when you start it. That's not exactly a secret or something unexpected. It's why ephemeral storage is wiped clean and why you can only retain data via ebs (which is not local storage).

If you restart an instance (rather than start/stop) then you stay on the same hardware.