If you haven’t updated your firmware, it is a know issue that the “L/Mono” output jack is very much “L” but very much not “Mono.” The whole summed to mono thing never worked from the get-go – the pedal remained stereo – and was only resolved a year or so ago with a firmware update. So anything panned hard right would all but disappear in the left channel.
(See No MONO Output for an example)