e.g. if src is:
srcRec := RECORD
DATASET(childrec) kids;
END;

and destination is:
dstRec := RECORD
childrec kids;
END;

This should be an error, but instead the target fields are filled by defaults, and the source is dropped.

Whenever the target is a nested record like this, and the source isn't, it should be an error.
Q: what about the case where the source contains all the fields (or some) of the nested record?