The ArcGIS GIS Server default printing service fails to output a legend symbology if the operational layer is a hollow polygon symbolized by only an outline.
This is a known limitation with the print service published from ArcMap. Beginning in ArcGIS GIS Server 10.6, the default print service is based on the ArcGIS Pro print engine and this behavior has been resolved.
解决办法
Migrate to the default print service running on ArcGIS Server 10.6 or newer
Publish a print service from ArcGIS Pro to ArcGIS Server 10.6 or newer.