Filters in NPrinitng apply selections in Qlik Sesnse data model. If you look close to the way how i suggest using variable you will notice that it starts with "=" sign, meaning that it evalueates every time there is a change in data model (change like applied selection etc.). So as soon as you apply NPrinitng filter it applies the field selection on Qlik Sense data model, variable gets evaluated, evaluated value gets returned back to NPrinting and can be used for whatever purpose you need it.
cheers Lech, When applicable please mark the correct/appropriate replies as "solution" (you can mark up to 3 "solutions". Please LIKE threads if the provided solution is helpful to the problem.