Cluster name on old and new nodes in 3.x / 4.5 mixed cluster doesn't seem to refer to the same key in the config

Steps to reproduce:

1. create a mixed cluster of 3.x and 4.5 nodes
2. update the cluster name on the 4.5 nodes; notice that it doesn't show on the 3.x nodes
3. update the cluster name on the 3.x nodes; notice it doesn't show on the 4.5 nodes

Looks like it's a different key or else the config replication is busted. I haven't looked into it; just filing so I don't forget the issue.