So the reason why this is so is that the rear connector only provides access to one CANbus, which is the Vehicle Bus. The display works by sending temperature commands directly to the VCRIGHT domain controller where the HVAC control lives (and actually it will accept temps .1C increments). The CAN signals for the UI display for temp are on the Ethernet bus which is deep in the dash connecting the MCU directly to the gateway. Since the rear display basically sends the same commands that the MCU would, and there is no complementary message for VCRIGHT to update the UI, the UI doesn't display the set temp, as the engineers designing the system understandably didn't take into account third party CAN injection devices when designing the messaging protocols. The reason why you have to manually set things like time zone and units is that these signals exist on the Chassis Bus, which is harder to access, with different solutions based on market and build date.