Issue with templates for system indices

We have recently upgraded to 1.12 and are having an issue with .opendistro-ism-managed-index-history indices including a few other system ones. We are unable to change the replicas. We were using the old template system which was working fine up until the upgrade (were on 1.8). As this broke we tried using the new template system however this does not work either. Here is the new template we created;

{
  "index_patterns": [
    ".opendistro-ism-managed-index-history-*"
  ],
  "template": {
    "settings": {
      "number_of_shards": 1,
      "number_of_replicas": 0,
      "index": {
        "opendistro.index_state_management.policy_id": "30days-delete"
      }
    }
  },
  "priority": 500,
  "version": 2,
  "_meta": {
    "description": "Fix the replica issue"
  }
}

This is happening on all our clusters. I was looking at Settings - Open Distro for Elasticsearch Documentation that says this is the correct method (It also needs correcting as it is missing the “template” : {} part)
I have used the index_template simulator and that shows the result we want, however this is not what is happening.

Thanks,

Tony.

FYI on the documentation issue: