I have reproduced this and am digging in further.
I can see, in the charm logs on Vault, that
2019-02-05 07:48:07 INFO juju-log certificates:3: Processing certificate request from remote-2ea1e7247f6746568fd0e43cacfdc7c0/0 for 10.5.0.11
where the above IP is the etcd unit in the remote model.
I have reproduced this and am digging in further.
I can see, in the charm logs on Vault, that
2019-02-05 07:48:07 INFO juju-log certificates:3: Processing certificate request from remote- 2ea1e7247f67465 68fd0e43cacfdc7 c0/0 for 10.5.0.11
where the above IP is the etcd unit in the remote model.