As an FYI, my headphone ports on my 18T don’t work either. Did all the troubleshooting, headphones work on guitar amp, tried ‘listening’ and ‘Main L/R’, no channels were solo. Reached out to the US distributor and they are going to RMA the unit after a factory reset didn’t work. I found a guy on another facebook group with the same problem on a 12. He RMA’d also.
There may be a build QA issue here. The mixer is working well otherwise.