But it’s true, it’s very strange.
The MIDI Learn CC 6 is recognized in Carla, Reaper, and Renoise’s MIDI monitor, but not in GMIDIMonitor, Ardour, or Mixbus.
It’s not a big deal since we can change the controller mapping, but it’s surprising that it works with some software and not others.
Out of curiosity, I also checked gmidimonitor (v3.5 on Debian Trixie), and it captures CC6 regardless of the backend used. Have you tried both Jack and ALSA backends?
Yeah, probably. But who knows what else may not work…