Unable to create a TrustFrameworkKeySet and TrustFrameworkKey at the same time

Describe the bug

I am trying to create a TrustFrameworkKeySet and TrustFrameworkKey at the same time using Microsoft.Identity.Web.GraphServiceClientBeta version 3.0.1 which is using Microsoft.Graph.Beta 5.78.0-preview. I get the following error:
The 'keySet' field is invalid in request. Please check the request body and parameters.

Expected behavior

To create a TrustFrameworkKey while creating a TrustFrameworkKeySet using the follow guide trustframework-post-keysets.

How to reproduce

Where:
secret - the key to add
name - name of the key to create
graphServiceClient - Microsoft.Graph.Beta.GraphServiceClient

TrustFrameworkKeySetCollectionResponse? keySets = await graphServiceClient.TrustFramework.KeySets.GetAsync(cancellationToken: cancellationToken);

TrustFrameworkKeySet? trustFrameworkKeySet = keySets?.Value?.FirstOrDefault(x => x.Id == name);

if (trustFrameworkKeySet == null)
{
    trustFrameworkKeySet = new()
    {
        Id = name,
        Keys =
        [
            new TrustFrameworkKey()
            {
                Use = "sig",
                K = secret,
                Kty = "oct"
            }
        ]
    };

    trustFrameworkKeySet = await graphServiceClient.TrustFramework.KeySets.PostAsync(trustFrameworkKeySet, cancellationToken: cancellationToken);
}

SDK Version

5.78.0-preview

Latest version known to work for scenario above?

No response

Known Workarounds

I am able to create the TrustFrameworkKeySet then Upload a secret separately but it always creates a .bak key.

Debug output

</details>


### Configuration

_No response_

### Other information

_No response_

Start with the TrustFramework.KeySets.PostAsync call and compare its TrustFrameworkKeySet payload with the linked trustframework-post-keysets example for Microsoft.Graph.Beta 5.78.0-preview. Investigate why the keySet field is rejected when a key is included during creation, then verify that creating the set with the requested key succeeds without producing an unexpected .bak key.