I am not seeing what Brendan is mentioning in #1.  I see this:

root@maas-dev:~# curl -s -k -u admin:admin -X GET https://10.3.0.1:8443/redfish/v1/Nodes
{
  "@odata.context" : "/redfish/v1/$metadata#Nodes",
  "@odata.id" : "/redfish/v1/Nodes",
  "@odata.type" : "#ComposedNodeCollection.ComposedNodeCollection",
  "Name" : "Composed Nodes Collection",
  "<email address hidden>" : 4,
  "Members" : [ {
    "@odata.id" : "/redfish/v1/Nodes/226"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/255"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/256"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/257"
  } ],
  "Actions" : {
    "#ComposedNodeCollection.Allocate" : {
      "target" : "/redfish/v1/Nodes/Actions/Allocate"
    }
  }
root@maas-dev:~# maas admin pods create type="rsd" power_address="10.3.0.1:8443" power_user="admin" power_pass="admin"
Success.
Machine-readable output follows:
{
    "architectures": [
        "amd64/generic"
    ],
    "used": {
        "local_disks": 4,
        "memory": 124992,
        "cores": 176,
        "local_storage": 479999999996
    },
    "total": {
        "local_disks": 7,
        "memory": 249984,
        "cores": 352,
        "local_storage": 959999999999
    },
    "name": "loving-bison",
    "type": "rsd",
    "resource_uri": "/MAAS/api/2.0/pods/190/",
    "available": {
        "local_disks": 3,
        "memory": 124992,
        "local_storage": 480000000003,
        "cores": 176
    },
    "id": 190,
    "capabilities": [
        "composable",
        "fixed_local_storage"
    ]
}
root@maas-dev:~# maas admin pods read
Success.
Machine-readable output follows:
[
    {
        "total": {
            "local_storage": 959999999999,
            "cores": 352,
            "memory": 249984,
            "local_disks": 7
        },
        "available": {
            "cores": 176,
            "local_disks": 3,
            "memory": 124992,
            "local_storage": 480000000003
        },
        "id": 190,
        "architectures": [
            "amd64/generic"
        ],
        "name": "loving-bison",
        "type": "rsd",
        "capabilities": [
            "composable",
            "fixed_local_storage"
        ],
        "used": {
            "local_storage": 479999999996,
            "cores": 176,
            "memory": 124992,
            "local_disks": 4
        },
        "resource_uri": "/MAAS/api/2.0/pods/190/"
    }
]
root@maas-dev:~# for i in {1..4}; do maas admin pod compose 190; done
Success.
Machine-readable output follows:
{
    "system_id": "spfkee",
    "resource_uri": "/MAAS/api/2.0/machines/spfkee/"
}
Success.
Machine-readable output follows:
{
    "system_id": "mftwep",
    "resource_uri": "/MAAS/api/2.0/machines/mftwep/"
}
Success.
Machine-readable output follows:
{
    "resource_uri": "/MAAS/api/2.0/machines/dgx734/",
    "system_id": "dgx734"
}
Success.
Machine-readable output follows:
{
    "system_id": "7ygnk7",
    "resource_uri": "/MAAS/api/2.0/machines/7ygnk7/"
}
root@maas-dev:~# curl -s -k -u admin:admin -X GET https://10.3.0.1:8443/redfish/v1/Nodes
{
  "@odata.context" : "/redfish/v1/$metadata#Nodes",
  "@odata.id" : "/redfish/v1/Nodes",
  "@odata.type" : "#ComposedNodeCollection.ComposedNodeCollection",
  "Name" : "Composed Nodes Collection",
  "<email address hidden>" : 8,
  "Members" : [ {
    "@odata.id" : "/redfish/v1/Nodes/226"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/255"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/256"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/257"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/263"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/264"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/265"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/266"
  } ],
  "Actions" : {
    "#ComposedNodeCollection.Allocate" : {
      "target" : "/redfish/v1/Nodes/Actions/Allocate"
    }
  }
root@maas-dev:~# maas admin pod delete 190
Success.
Machine-readable output follows:

root@maas-dev:~# curl -s -k -u admin:admin -X GET https://10.3.0.1:8443/redfish/v1/Nodes
{
  "@odata.context" : "/redfish/v1/$metadata#Nodes",
  "@odata.id" : "/redfish/v1/Nodes",
  "@odata.type" : "#ComposedNodeCollection.ComposedNodeCollection",
  "Name" : "Composed Nodes Collection",
  "<email address hidden>" : 4,
  "Members" : [ {
    "@odata.id" : "/redfish/v1/Nodes/226"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/255"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/256"
  }, {
    "@odata.id" : "/redfish/v1/Nodes/257"
  } ],
  "Actions" : {
    "#ComposedNodeCollection.Allocate" : {
      "target" : "/redfish/v1/Nodes/Actions/Allocate"
    }
  }
}root@maas-dev:~# 


As this shows, the nodes are being decomposed once the pod is deleted.  However, if I do delete the machines manually with MAAS before deleting the Pod I do see that they are not decomposed, which is what is mentioned in the bug description.