I do not think your keyboard is defective. Because the polyphonic aftertouxh uses same sensor with velocity, so if you can receive midi note, the aftertouch feature could not be broken.
Please be aware the polyphonic aftertouch in Xkey is different than monophonic aftertouch from any other keyboard, it is more professional feature and only Xkey has it today. You can simple test with a midi monitor software to see that you can receive it from the Xkey. So the only issue is your sound selection which can respond this message.