According to https://github.com/cloudevents/spec/blob/main/cloudevents/formats/json-format.md#311-payload-serialization,

If the datacontenttype declares the data to contain JSON-formatted content, a JSON serializer MUST translate the data value to a JSON value, and use the member name data to store it inside the JSON representation. The data value MUST be stored directly as a JSON value, rather than as an encoded JSON document represented as a string. An implementation MAY fail to serialize the event if it is unable to translate the runtime value to a JSON value.

However, the WriteJson() function in in event_marshal.go only consider ApplicationJSON and TextJSON (and also default empty) data content type as isJson=true.

https://github.com/cloudevents/sdk-go/blob/main/v2/event/event_marshal.go#L153

It results in writing "structured" type ApplicationCloudEventsJSON as a string, not a JSON value.

https://github.com/cloudevents/sdk-go/blob/main/v2/event/event_marshal.go#L172