Official comment
I understand - and their response is that unfortunately the XDJ-XZ was designed as a stand-alone unit with the ability to control rekordbox performance mode, not as a rekordbox controller as its primary function. As such, some features won't operate as you may find on other devices (such as a controller), whose audio is actually mixed internally. As a result, the pad FX can not be operated as post-fader, but the built-in beat FX on the XZ are pretty amazing (and they can be used post-fader).