The way NPrinting is constructed, it presumes that you only install a single server installation where files for a single environment will be stored.
So yes this is by design.
The purpose of relative paths is to ensure smooth movement of files between two separate computers with similar drive path with with separate root drives.
TestServer - c:\nprinting\nprintingtemplates
ProdServer - f:\nprinting\nprintingtemplates
relative path on both computers ..\nprinting\nprintingtemplates
I would recommend a separate test server. You may contact your Qlik account manager for details.