@peterjones than you for the answer.
I agree that standard Windows associating methodology can cover a major number of usecases when working with text files. But sometimes Windows or some apps open files for you without asking what editor to use (under the hood they call default notepad in similar way like I specified above).
It looks like the parameter -notepadStyleCmdline was created specifically for this “hack” with replacing default notepad, but there is something wrong with parameter processing in NPP.