There is an issue with 4.1 when it looses focus from the screen and when you have debug buttons enabled. Disable the debug buttons and see if it works.
Given there is no documentation on APD under RhoElements v4.1 I would presume it's not supported so shifting strategy to RhoElements ZebraPrinter APIs as per doc outlined here Rhomobile | PrinterZebra
However the doc covers only the BlueTooth Connection Type.
Has anyone acquired knowledge on establishing a serial connection to a cable tethered printer???