7.2.5.4 DataSetMessage
There is text that defines special handling on top level:
If the FieldEncoding1 is FALSE in the DataSetMessageContentMask, the Variant at the top level of a field is encoded as a JSON value containing only the value of the Body field. If this Variant contains an ExtensionObject, the ExtensionObject shall be encoded as a Structure without the UaTypeId field. This also applies to the Variant in a DataValue at the top level of a field.

This does not cover the special cases of having fields with abstract type or allow subtype.
We need to describe all special cases