You cant see the decorated field values, looks like decorator cannot apply on the indexed stream and it only work with search result, is it true ? what is the real reason we are unable to support on the indexed result ?
have you added the decorator to the stream you are linking to as well? The decorator configuration is persisted per stream, so when you link to the other stream from the alerting mail, the decorated fields simply do not exist because the decorator is not run for that stream.