openstack stack update does not replace a failed node

## my stack

$ nova list
host-0.example.com  192.168.0.5
host-1.example.com  192.168.0.6

hosts are

  - in a OS::Heat::ResourceGroup
  - in a ServerGroup with an anti-affinity policy

## reproduce

Delete one node
$ nova delete host-1.example.com
$ nova list
host-0.example.com

Stack check correctly detects the missing node (see paste)
$ heat action-check mystack

Run stack-update to recover the missing node
$ heat stack-update -x mystack


## I expect

The stack to be recovered to the original state

## Instead
No more nodes are created.

$ nova list
host-0.example.com 192.168.0.11