Thanks for the hint! I’ve quickly checked previous versions of MP, and it works well up to v1.3.52.
1.3.53 and the latest 1.3.55 do not show the traffic anymore. Looks like a bug unless there is a hidden option to enable it somehow.
Regarding the ADSB checkbox, it doesn’t have to be enabled for pingRX to work. It seems that this option only imports traffic from external sources such as dump1090.
See here https://discuss.ardupilot.org/t/ads-b-mission-planner-and-flightaware-ads-b-dongle/21362/9