Wing fc-10 --> Walksnail OSD support?

Hi,
I am trying to get the OSD for a caddx walksnail system working on a wing fc-10. I was following the instructions on:
https://ardupilot.org/plane/docs/common-msp-osd-overview-4.2.html#id2
I am using the pins labelled rx1 and tx1:
https://ardupilot.org/plane/docs/common-furiousfpv-f35.html
These should correspond to serial1.
But I do not manage to get any OSD (tried swapping pins, and also tried other serial ports).

Is there maybe a hardware limitation that I am overlooking…?

Thanks for your input!

Solved with a pulldown on the FC tx pin: