OpenAPI: Required method parameters become nullable with /GenerateOptionalParameters:true

[bkoelman]

When adding the /GenerateOptionalParameters:true option in NSwag generated clients, the request body parameter becomes nullable. For example:

public virtual async Task<ApiResponse<CountryPrimaryResponseDocument>> PatchCountryAsync(string id,
    IDictionary<string, string?>? query = null, CountryPatchRequestDocument? body = null,
    CancellationToken cancellationToken = default(CancellationToken))

This is incorrect because it always fails with HTTP 422.

The OAS for this method looks correct, as far as I'm aware:

"requestBody": {
  "description": "The attributes and relationships of the country to update. Omitted fields are left unchanged.",
  "content": {
    "application/vnd.api+json": {
      "schema": {
        "allOf": [
          {
            "$ref": "#/components/schemas/countryPatchRequestDocument"
          }
        ]
      }
    }
  }
}

I've opened this issue to investigate why this is happening.