Comment 3 for bug 1773117

So the stacktrace ends up being:
machine-0: 19:04:28 TRACE juju.apiserver.common server RPC error [
{github.com/juju/juju/apiserver/common/networkingcommon/networkconfigapi.go:192: }
{github.com/juju/juju/apiserver/common/networkingcommon/networkconfigapi.go:78: cannot get network interfaces of "4s33ky"}
{github.com/juju/juju/provider/maas/interfaces.go:330: }
{github.com/juju/juju/provider/maas/environ.go:1756: instance "4s33ky" not found}
]

and that line is specifically:
 if len(instances) == 0 {
  return nil, errors.NotFoundf("instance %q", instId)
 }

So somehow:
func (environ *maasEnviron) getInstance(instId instance.Id) (instance.Instance, error) {
 instances, err := environ.acquiredInstances([]instance.Id{instId})

with that instance id, really is returning no instances.