Pixhawk 4 with PingRX ADS-B Receiver does not work

I am using a Pixhawk 4 with ArduPlane 3.9.1 installed and connected my PingRX receiver to the Telem2 port.
In the configuration tab I set up SERIAL2_BAUD to 57 and SERIAL2_PROTOCOL to 1.
I also enabled ADSB_ENABLE and rebooted the Pixhawk.
Before I switched to the Pixhawk 4, I used a Pixhawk with the PingRX and it worked flawlessly. Now with the Pixhawk 4, I see the LED is blinking when planes are in the near, but I cannot see any planes in Mission Planner (1.3.60).
Does someone have an idea what is wrong with my setup?

I found a solution to this: if I connect the PingRX to the UART & I2C B port and change the SERIAL4_BAUD to 57 and the SERIAL4_PROTOCOL to 1 it seems to work. I have no idea why it does not work with the same settings on the TELEM2 port.