The stretch version of scp displays fingerprints using a SHA256 hash, but of course a jessie machine has no idea of what its fingerprint is except using an MD5 hash. A question and suggestion:
Is there a way to get stretch scp to show the fingerprint in MD5 when connecting to a new (jessie) host? IIRC the debian-installer only shows the SHA256 fingerprint. It ought to display the MD5 version too, for those people installing over the network with ssh from an older machine. Cheers, David.