This is a known limitation of print services (as of version 10.5). When adding a hosted feature service to the Portal for ArcGIS map viewer, printing a map with a legend causes labels in the legend to be displayed as 'Override X' instead of the correct label names.
This behavior also occurs if the symbology is changed in the map viewer or when transparency is set. When the layer does not have the default symbology, the print service considers the symbol change (or transparency) an override, and sets the legend labels to 'Override X'. As a workaround, use ArcMap or ArcGIS Pro to set the desired symbology before publishing and printing the map.
Note: This issue may also occur when printing via the Print widget in Web AppBuilder for ArcGIS applications, or when printing in the Basic Viewer application template.