On your separate mixers, go to channel type and UNCHECK the type of channels you do not want to see in that particular mixer. That should solve your problem except I’m not sure about mixer 1. There are known problems with mixer 1 so I’m using only mixer 2 and 3. (3 screens on my set up.)
Also, look at this thread I started which Steinberg has not responded to.
This is similar to your issue, but has to do with placing the new added channels in the proper group folder instead of automatically defaulting to the left zone.