It seems this might be a juju 1 vs. juju 2 incompatibility; I compared with a previously-deployed version of the charm, and the only difference was in the saved persistent config: in the juju 1 config, "registration-key" was absent, whereas in juju 2 it was set to the empty string. I'm not sure if this is really a bug or should be considered a configuration error on my part, but it certainly is surprising.
It seems this might be a juju 1 vs. juju 2 incompatibility; I compared with a previously-deployed version of the charm, and the only difference was in the saved persistent config: in the juju 1 config, "registration-key" was absent, whereas in juju 2 it was set to the empty string. I'm not sure if this is really a bug or should be considered a configuration error on my part, but it certainly is surprising.