With the model:

  container container {
    list list {
      key "name address";
      leaf name {
        type string;
      }
      leaf address {
        type string;
      }
    }
  }

We are getting two POST requests to create list entry:

1. /rests/data/network-topology:network-topology/topology=topology-netconf/node=17830-sim-device/yang-ext:mount/test:container with payload:

{
  "list": [
    {
      "name": "Some name",
      "address": "Some address"
    }
  ]
}

Which works fine and is in accordance with general logic that POST request creates child resource.

2. /rests/data/network-topology:network-topology/topology=topology-netconf/node=17830-sim-device/yang-ext:mount/test:container/list={name},{address} with payload:

{
  "name": "Some name",
  "address": "Some address"
}

Which fails with: Error parsing json input: Failed to create new parse result data. Are you creating multiple resources/subresources in POST request?

Remove the option #2 even if it works for one item keys.