In an application as complex as Dorico, it’s just not possible to document every caveat, or provide UI and warning messages for every possible situation that a user may encounter. Every extra option and error message requires substantial work to implement, test and localise into 9+ languages. We try to find ways to minimise ambiguities, but this isn’t always possible. Especially as the application is developing at a very rapid rate, and it’s not until users have been using the application for a while before patterns of usage emerge. To many users familiar with DAWs or other notation packages, it’s expected that a MIDI mixer will send the MIDI volume and pan controls (CC7, CC10).
Longer term we anticipate that the mixer and automation lanes will be tied more closely together. For the moment though, you can add a CC automation lane to control CC7 over time. Any automation you draw there will prevent the mixer value from being sent.