FrSky R-XSR Fport telemetry with potentiometer

Fport telemetry from the flight controller (a PlayF4 with an F411 in this case) won't work if the resistor between the TX pin and the R-XSR is bigger than 60 ohms.
In the first second you can see that all sensors work (blinking asterixis after all sensors). When I increase the resistance by turning the potentiometer the telemetry from the FC disappears (sensors 3, 4 and 5), while the telemetry from the R-XSR itself is still working (sensor 1 and 2).
I happen to have an FC where the built in resistor between the UART and the pad is 220 ohm. I was wondering why I couldn't get the F.port telemetry working. Bridging the resistor fixed it.
Mateksys is also aware of this issue, see for example www.mateksys.co....