AWS Cost Explorer API doesn't return Resource ID

I'm testing the AWS Cost Explorer API (I'm using the .NET SDK), in particular the GetCostAndUsageWithResources method to get the costs split by resource.

This is the code I'm testing with:

string nextPageToken = null;

do
{
    var costRequest = new GetCostAndUsageWithResourcesRequest()
    {
        Granularity = Granularity.HOURLY,
        GroupBy = {
            new GroupDefinition() {
                Key = "RESOURCE_ID",
                Type = GroupDefinitionType.DIMENSION
            }
        },
        Metrics = { "BlendedCost" },
        NextPageToken = nextPageToken
    };

    var costResponse = await client.GetCostAndUsageWithResourcesAsync(costRequest);
    nextPageToken = costResponse.NextPageToken;

    foreach (var resultByTime in costResponse.ResultsByTime)
    {
        foreach (var instanceGroup in resultByTime.Groups)
        {
            var instanceId = instanceGroup.Keys.First();
            if(g.Keys.First() != "NoResourceId" && !g.Keys.First().StartsWith("i-"))
            {
                Debugger.Break(); //NEVER gets hit
            }
        }
    }

} while (!string.IsNullOrEmpty(nextPageToken));

However, as you can see from the comment in the code, I have an issue: the resource ID (which is the dimension I'm grouping by) seems to only be retrieved correctly for EC2 machine instances (IDs that start with i-). Otherwise, all other results have the ID key set to NoResourceId

What am I doing wrong here? Why does the Cost Explorer API only populate the Resource ID of EC2 instances, and all others are not identified? What if I want to know the costs of all other AWS services, how do I identify to which service the result belongs?

Am I doing something wrong here in the way I invoke the API? What am I missing?