DocumentDB ReplaceDocument Fails

alternatively you can tell Cosmos to use camelCasingWhichIsStandardForJson

new CosmosClient(
  connectionstring,
  new CosmosClientOptions(){
    SerializerOptions = new CosmosSerializationOptions(){
      PropertyNamingPolicy = CosmosPropertyNamingPolicy.CamelCase
    }

  }
)

OK I figured it out.

Each Document in DocumentDB needs to have an "id" property. If a class does not have one, it will get assigned one and saved into the document. With DocumentDB being case sensitive, my "Id" was just another property and a separate "id" was added and assigned to the document.

I fixed the issue by deleting and recreating all my documents with the following attribute for Id:

[JsonProperty(PropertyName = "id")]
public Guid Id { get; set; }