Juju scp reports "Host key verification failed. lost connection" when transferring files between machines, although is happy to connect to them individually

I’m having difficulty troubleshooting an error I’m receiving from the juju scp command when transferring files between machines on the same model.

$ juju scp 0:hello.txt 1:hello-txt
ERROR exit status 1 (Host key verification failed.
lost connection)

Using the --proxy option to proxy traffic via the controller doesn’t seem to help:

$ juju scp --proxy 0:hello.txt 1:hello-txt
ERROR exit status 1 (Host key verification failed.
lost connection)

I am able to conduct the transfer by injecting options directly to scp:

$ juju scp -- -3 0:hello.txt 1:hello-txt
$ echo $?
0

Any advice for diagnosing the underlying issue(s)?