Unable to serialize complex (custom) extension in the expected json format

creating a custom extension object implementing CloudEventExtension, when using the extension (customExt) as below, and then serializing the CloudEvent, the serialization for the CloudEvent is failing with JsonMappingException.

CloudEventBuilder.v1()
        .withSource(URI.create(source))
        .withType(event_type)
        .withTime(event_time)
        .withId(event.getEventId())
        .withDataContentType(contentType)
        .withData(eventPayaload)
        .withDataSchema(URI.create(XDM_SCHEMA_URI))
        .withExtension(DATA_SCHEMA_VERSION_KEY_EXTENSION_NAME, schemaVerExt)
        .withExtension(X_ACTION_ID_KEY_EXTENSION_NAME, requestIdExt)
        .withExtension(customExt);

here is how the extension looks like. The customExt is the Recipient object

Also, if using the other supported methods withExtension (with key, value) as below, I had to send the string version of the customExt, and then if serialized the output is not in the expected format (see below). It can break our customers event
integrations. Kindly suggest.

expected serialized CloudEvent custom extension

"recipient":{"userid":"9CE048DB5E8F485XXXXXXXXX@AdobeOrg"}

actual serialized CloudEvent custom extension

"recipient":"{\"userid\":\"9CE048DB5E8F485XXXXXXXXX@AdobeOrg\"}"

The serialization is failing while reading extensions here, as I believe the Object type value is not supported.

Start at core/src/main/java/io/cloudevents/core/impl/BaseCloudEvent.java around line 84, where extension values are read during serialization. Reproduce the issue with the custom Recipient extension and the withExtension overloads shown in the report, then compare the serialized output with the expected nested recipient JSON. Done means custom extension objects serialize without JsonMappingException and are not emitted as escaped JSON strings.