Event Registry Payload Json definition Type Error

We have setup Flowable v6.8 event registry with Kafka. We have a simple channel for inbound and outbound event. The inbound channel topics has all the required fields to do with instance, correlation and tenant. We have another field which is stored as a json value ( as it can contain custom parameters unknown at compile time. ):

“UserId”: “string”,
“TenantId”: “string”,
“CorrelationId”: “string”,
“DataObject”: “json”

The message gets received and can be viewed by kafka UI interface. In flowable-rest service we can see the logs throw an error when trying to do the JsonPayloadExtraction and seems to not know what type the DataObject is.

o.f.e.i.p.JsonFieldToMapPayloadExtractor : Unsupported payload type: null

From Flowable side we set the output event payload properties and map as follows:
variable name: DataObject
type: json
value expression: ${DataObject}

The flowable modeler UI only allows you to set either string, integer, double and boolean. From other forum posts we have concluded that you can manually set the sourceType and Target Type to json. Which from Flowable v6.6 it should understand.

We have also set in the .event file the DataType for DataObject is “json” , we have also tried the use of “Object”.

When trying to reference the variable within flowable expression the value is just empty. Example “${DataObject.Value}”. I am assuming this is to do with the fact it failed to extract the DataObject from the event payload and hence it would be empty.

Does anybody know how we define a custom event payload where the properties inside the payload can be of type json as well, as I can see this is supported by the event registry.
Am I missing a configuration file or setting somewhere?