I ran into a similar issue with the "Download source data" button failing to actually download the data, it would either give an error or just fail silently. I had also renamed the hosted feature sub-layer from its originally published name. This layer name had a dash "-" and a forward slash, "/". I renamed the layer to remove both of these characters, I don't think this was back to its original name, and now the Download button works! This map packaged fine for inclusion in offline areas, so whatever logic that the Dashboard uses for its download/packaging call must not properly account for special characters in the sub-layer name.
