The culprit is the custom RDP parameter /novcast when the SDA is connected.
If the customer removes this /novacast parameter in the RX-RDP configuration then the SDA would work properly.
Just to give you some background related to the current architecture:
The VNC shadowing in RX300 or RX-RDP, user can view Device’s Home Screen, Settings GUI only.
User won’t be able to view Desktop Session connected on the device. This limitation is due to the fact that the session display is drawn using Dispmanx. But, Dispmanx layers are not visible on VNC shadowing.
To view session running on RX300 or RX-RDP Devices, use /novcast parameter in RDP, VERDE RDP settings.
However, the only way for RX300/RX-RDP communicate with the SDA (based on Pi Zero) for the 2nd display must be based on Dispmanx. By having the /novast RDP parameter, it essentially disabled the 2nd screen display to the SDA.
If the customer wants to shadow the session with VNC, they can use the build-in VNC utility, with the /novcast RDP parameter. This works well in single display mode.
However, if there is dual display with our Pi Zero SDA connected, VNC shadow would not be possible, and this /novacast RDP parameter must be removed, otherwise, the second display will not be shown.