[2.4.1] juju doesn't allocate a subordinate if a primary has a failed hook pending resolution

I have been trying to add a subordinate just to prove that a missing subordinate causes a hook error due to a bug in the charm:
https://bugs.launchpad.net/charm-rabbitmq-server/+bug/1786164

So I did:

juju deploy nrpe --bind "oam-space"
juju relate rabbitmq-server nrpe

rabbitmq-server/0* error idle 2/lxd/5 10.232.6.197 hook failed: "config-changed"

Relating mysql to nrpe immediately leads to a subordinate allocation:

juju relate mysql nrpe

mysql/0* active idle 1/lxd/2 10.232.6.212 3306/tcp Unit is ready
  nrpe/0 waiting allocating 10.232.6.212 installing agent

juju status --relations nrpe | grep nrpe-external
mysql:nrpe-external-master nrpe:nrpe-external-master nrpe-external-master subordinate
rabbitmq-server:nrpe-external-master nrpe:nrpe-external-master nrpe-external-master subordinate joining

Resolving the rabbitmq-server/0 unit temporarily does not help.

Update:

juju remove-machine --force 2/lxd/5
juju add-unit rabbbitmq-server --to lxd:2

# still no subordinate created, even for a new unit

rabbitmq-server/1* error idle 2/lxd/6 10.232.6.196 hook failed: "config-changed"

# worked around the charm bug
root@juju-06074d-2-lxd-6:~# mkdir -p /usr/local/lib/nagios/plugins/

juju resolved rabbitmq-server/1

# now, after rabbitmq-server is good, the subordinate is finally allocated

rabbitmq-server/1* active executing 2/lxd/6 10.232.6.196 5672/tcp,15672/tcp Unit is ready
  nrpe/1 maintenance executing 10.232.6.196 (install) starting