Unable to re-expose the controller charm after unexposing it

Hey team,

In my controller model, my controller charm was exposed by default. When I did juju status, I could see yes under column Exposed.

I was following the expose and unexpose docs.
After successfully "unexposing" my controller, I wanted to expose it again. I ran juju expose controller, which gave me the following error:

ERROR cannot expose a container application without a "juju-external-hostname" value set, run
juju config controller juju-external-hostname=<value>

However, my controller config nevere had juju-external-hostname set to begin with. If I run juju config juju-external-hostname, I don't see it there.

My questions are:

• How was the controller charm exposed in the first place without a juju-external-hostname?
• If this value was never set and yet exposing the controller worked before, how come I'm not able to re-expose the controller again with an empty value?
• If I set the value to some made up value e.g. "foo", the exposition works. Why?
• Also, can I ask why it's necessary for the controller to be exposed by default.
  Thank you!

Related issues:

• https://github.com/canonical/opentelemetry-collector-k8s-operator/issues/132

Start with the expose and unexpose documentation linked in the issue, then trace the behavior of juju expose controller and juju config controller juju-external-hostname. Compare the controller's initial exposed state with its state after unexposing, including the empty configuration case. Done means the cause and expected behavior are established and the issue is corrected or documented clearly.