2.7.6/2.8.0: subordinates get empty relation data from relation-get

cs:telegraph-6 fails juju-info-relation-joined hook with 2.7.6, but not with 2.6.10.

with 2.7.6 and 2.8.0:
$ juju run --unit telegraf/0 -- 'relation-get -r 0 - ubuntu/0'
{}

with 2.6.10:
$ juju run --unit telegraf/0 -- 'relation-get -r 0 - ubuntu/0'
egress-subnets: 10.62.162.107/32
ingress-address: 10.62.162.107
private-address: 10.62.162.107

unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined Traceback (most recent call last):
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined File "/var/lib/juju/agents/unit-telegraf-0/charm/hooks/juju-info-relation-joined", line 19, in <module>
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined main()
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.6/site-packages/charms/reactive/__init__.py", line 78, in main
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined bus.dispatch()
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 423, in dispatch
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined _invoke(other_handlers)
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 406, in _invoke
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined handler.invoke()
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined File "/var/lib/juju/agents/unit-telegraf-0/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 280, in invoke
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined self._action(*args)
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined File "/var/lib/juju/agents/unit-telegraf-0/charm/reactive/telegraf.py", line 221, in configure_telegraf
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined if get_remote_unit_name() is None:
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined File "/var/lib/juju/agents/unit-telegraf-0/charm/reactive/telegraf.py", line 114, in get_remote_unit_name
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined if rel['private-address'] == hookenv.unit_private_ip():
unit-telegraf-0: 20:09:53 DEBUG unit.telegraf/0.juju-info-relation-joined KeyError: 'private-address'
unit-telegraf-0: 20:09:53 ERROR juju.worker.uniter.operation hook "juju-info-relation-joined" failed: exit status 1