ArcGIS Open Data is unable to index metadata from the Item Details page when datasets are added at the service level and include more than one layer. Instead, the metadata is indexed from the Representational State Transfer (REST) page of each individual layer.
Option 1: Add the layers of the service to ArcGIS Online Content individually using the REST URL of each layer. The ArcGIS Open Data is able to read metadata from the Item Details page.
Option 2: Set the needed description in the layer Properties for each layer prior to publishing the service. The ArcGIS Open Data reads the description from the REST page of the layer as expected.