Why do people assume that shiny and white is the same as properly cleaned??
You need to LOOK at the filter and see if there is any collected soil, i dont give a rats arse how white nappisan makes it, you need to see, before you put nappisan near it, that there is no collected surface goo. Filters with plastic cages around them mean you cant, and i wouldn't buy one, ever, for that very reason.
Once it is actually "clean" on its surface, then worry about your nappisan etc. Sodium perc does a pretty good job... Good enough so i use it for about 3 or 4 brews worth. - then i give the filter a PBW soak, and surprie surprise - when you use a proper brewery detergent in it, all manner of stuff gets cleaned out that the nappisan didn't touch. - and the filter performane improves markedly.
Nappisan is an OK cleaner, but it just is NOT as good as a proper cleaner and if you study flitration... You'll want to use a proper cleaner.
Backflush, inspect, surface clean ( highish pressure spraying) if needed, soak in nappisan - rinse, sanitise, use.
Every three or four brews - same routine but replace nappisan with PBW - then nappisan to make it white and you feel better about it.