If I export an element with the hostname tag set:

In the csv, it has stored the data that was in the #Hostname field under the ScenarioSelectionSources heading:

If I then import this csv, the hostname tag now comes under the ScenarioSelectionSources:

And no longer in the #Hostname field:

The bug appears to be in the export function only. If I modify the exported csv so that the hostname data is in the correct comma-separated value, when I import it again the data returns to the #Hostname field. So import works correctly.