Comment 6 for bug 1885339

So it concretely look like:

'relation-list' is correctly reporting that the unit is gone but
'relation-get' still lets you read the old data, because that data isn't removed from the database.

Probably we are missing a cleanup step that ensures a departing unit's data is removed from the settings collections.

We could paper around it by having the Unit agent refuse to return the data if it isn't currently part of the units joined in the relation, but we should really fix it at the source.