Adding actions to iControlREST transaction

on my side it is working, using 11.6 here is the workflow

1) transaction creation : POST /mgmt/tm/transaction response :

{
   "transId": 1411771265430600,
   "state": "STARTED",
   "timeoutSeconds": 30,
   "kind": "tm:transactionstate",
   "selfLink": "https://localhost/mgmt/tm/transaction/1411771265430600?ver=11.6.0"
}

2) adding a command to transaction POST /mgmt/tm/ltm/pool

Header : X-F5-REST-Coordination-Id: 1411771265430600

Payload:

 {
"name":"transac-pool1",
"members": [ {"name":"192.168.25.32:80","description":"First pool for transactions"} ]
}

Response :

 {
   "method": "POST",
   "uri": "https://localhost/mgmt/tm/ltm/pool",
   "body":    {
      "name": "transac-pool1",
      "members": [      {
         "name": "192.168.25.32:80",
         "description": "First pool for transactions"
      }]
   },
   "evalOrder": 1,
   "commandId": 1,
   "kind": "tm:transaction:commandsstate",
   "selfLink": "https://localhost/mgmt/tm/transaction/1411771265430600/commands/1?ver=11.6.0"
}

i received a 200 OK but if you check the response code is slightly different from a traditional REST call.

3) second call for another pool is doing the same thing

4) having a look to the pending commands : GET /mgmt/tm/transaction/1411771265430600/commands

{
   "kind": "tm:transaction:commandscollectionstate",
   "selfLink": "https://localhost/mgmt/tm/transaction/1411771265430600/commands?ver=11.6.0",
   "items":    [
            {
         "method": "POST",
         "uri": "https://localhost/mgmt/tm/ltm/pool",
         "body":          {
            "name": "transac-pool1",
            "members": [            {
               "name": "192.168.25.32:80",
               "description": "First pool for transactions"
            }]
         },
         "evalOrder": 1,
         "commandId": 1,
         "kind": "tm:transaction:commandsstate",
         "selfLink": "https://localhost/mgmt/tm/transaction/1411771265430600/commands/1?ver=11.6.0"
      },
            {
         "method": "POST",
         "uri": "https://localhost/mgmt/tm/ltm/pool",
         "body":          {
            "name": "transac-pool2",
            "members": [            {
               "name": "192.168.25.32:80",
               "description": "second pool for transactions"
            }]
         },
         "evalOrder": 2,
         "commandId": 2,
         "kind": "tm:transaction:commandsstate",
         "selfLink": "https://localhost/mgmt/tm/transaction/1411771265430600/commands/2?ver=11.6.0"
      }
   ]
}

5) then submit transaction PATCH /mgmt/tm/transaction/1411771265430600

no header !

payload :

 { "state":"VALIDATING" }

response:

    {
   "transId": 1411771265430600,
   "state": "COMPLETED",
   "timeoutSeconds": 30,
   "kind": "tm:transactionstate",
   "selfLink": "https://localhost/mgmt/tm/transaction/1411771265430600?ver=11.6.0"
}

Having a look to GUI, pool is created.

Regarding you test, the second POST request should be correct, you create a POST as you would do usually but only the header F5-REST-Coordination-Id differentiate it from a normal request.