$ vnet=$(az rest -m GET -u /subscriptions/0b1f6471-1bf0-4dda-aec3-cb9272f09590/resourceGroups/my-test/providers/Microsoft.Network/virtualNetworks/vnettest?api-version=2021-02-01)

$ echo "$vnet"
{
  "etag": "W/\"6edecdf3-4fb4-4b9e-a3a0-bcf184d6caf7\"",
  "id": "/subscriptions/0b1f6471-1bf0-4dda-aec3-cb9272f09590/resourceGroups/my-test/providers/Microsoft.Network/virtualNetworks/vnettest",
  "location": "centralus",
  "name": "vnettest",
  "properties": {
    "addressSpace": {
      "addressPrefixes": [
        "10.1.0.0/16"
      ]
    },
    "enableDdosProtection": false,
    "provisioningState": "Failed",
    "resourceGuid": "069414bc-bd93-47a1-976e-2e51578a2c72",
    "subnets": [
      {
        "etag": "W/\"6edecdf3-4fb4-4b9e-a3a0-bcf184d6caf7\"",
        "id": "/subscriptions/0b1f6471-1bf0-4dda-aec3-cb9272f09590/resourceGroups/my-test/providers/Microsoft.Network/virtualNetworks/vnettest/subnets/default",
        "name": "default",
        "properties": {
          "addressPrefix": "10.1.0.0/24",
          "delegations": [],
          "privateEndpointNetworkPolicies": "Disabled",
          "privateLinkServiceNetworkPolicies": "Disabled",
          "provisioningState": "Failed"
        },
>>      "resourceGroup": "my-test",
        "type": "Microsoft.Network/virtualNetworks/subnets"
      }
    ],
    "virtualNetworkPeerings": []
  },
>>"resourceGroup": "my-test",
  "type": "Microsoft.Network/virtualNetworks"
}

$ az rest -m PUT -u /subscriptions/0b1f6471-1bf0-4dda-aec3-cb9272f09590/resourceGroups/my-test/providers/Microsoft.Network/virtualNetworks/vnettest?api-version=2021-02-01 --body "$vnet"
Bad Request({
  "error": {
    "code": "InvalidRequestFormat",
    "message": "Cannot parse the request.",
    "details": [
      {
        "code": "InvalidJson",
        "message": "Could not find member 'resourceGroup' on object of type 'Subnet'. Path 'properties.subnets[0].resourceGroup', line 27, position 24."
      },
      {
        "code": "InvalidJson",
        "message": "Could not find member 'resourceGroup' on object of type 'VirtualNetwork'. Path 'resourceGroup', line 33, position 18."
      }
    ]
  }
})