I was expecting to hear about someone who got a new phone, set up Signal using the phone number, and then communicated with contacts without any more authentication. This doesn't appear to be anything of the kind. Signal has implemented a migration system so that people can move to a new phone without changing the safety number. Then a bunch of references to the promises Signal makes about re-verifying if the safety…
I imagine there is a human factors tradeoff here. If signal notified on each migration, users with even medium sized contact lists would likely be getting constant notifications - making the alert useless. See medical device or aviaton alerts experiences.
It is like copying your PGP private keys from one device to another. The associated public keys are still in your correspondents key stores, certified or not. Nothing changes for them and there is no reason for them to know what you have done.
There is nothing here for Signal to do. Their issue is is the one that everyone who creates an encrypted E2EE messenger faces. There is no good analogy for cryptographic identities. So it is hard to produce a good conceptual model that the user can use to understand what they should expect and how they should react.