Duplicate of NIM087678.
This issue was fixed in ArcGIS GIS Server 10.6.
This issue is not reproducible if upgraded to ArcGIS GIS Server to 10.6 when using the out-of-the-box printing services (Utilities/Printing tools geoprocessing service) or publishing a new print service from ArcGIS Pro 2.1 and beyond to a ArcGIS Server 10.6.
The issue still occurs for an existing custom printing service created before from ArcMap or publishing a new printing service to an ArcGIS Server 10.6 using ArcMap 10.6.