It's rather disappointing UX and something that trains users to accept key changes of their contacts that Signal doesn't support affirmations of key continuity.
If you get a new phone without having lost the old one, it would be good to have a feature where Signal on the new phone shows its public key as a QR code, you scan it with Signal on the old phone and Signal on the old phone generates a protocol message to contacts indicating legitimate key roll-over without "key changed but you don't know why" UX.
If you get a new phone without having lost the old one, it would be good to have a feature where Signal on the new phone shows its public key as a QR code, you scan it with Signal on the old phone and Signal on the old phone generates a protocol message to contacts indicating legitimate key roll-over without "key changed but you don't know why" UX.