if you get
$ balena ssh e19f firstname.lastname@example.org: Permission denied (publickey). Warning: ssh process exited with non-zero code "255"
it might mean your balena cloud ssh key (https://dashboard.balena-cloud.com/preferences/sshkeys) is out of date
the fact that balena cli is otherwise working DOES not mean you SSH keys are up to date which is the mistake i was making
it might be worth documenting this somewhere maybe in the faqs