Cannot arm manually with joystick / throttle slider

First posted this in PX4 Software development but looks like it might be Qgroundcontrol related.

Had a look at this Github post as well

Tried all the different calibration options (centre, full down).

Have a prototype ground station setup with joystick and throttle slider which does not arm when throttle is full down and joystick is rudder full right.

Works well with Ardupilot. Trying to demonstrate compatibility with PX4.