Hi! With some help from MediaInfo it becomes clear, that Cubase exports mp3s using “joint stereo” technology, not true stereo. Some audio distributors does not accept joint stereo. So, there must be a way to change this behaviour. Where is that magic checkbox?
It is also the default behaviour for LAME encoder but you can override that. Say, in Audacity you are asked to choose one of a few stereo modes. In ffmpeg the user is given -joint_stereo=0 command line argument. I was highly surprised to see the lack of it in Cubase, being unable to do what I need.