Right now, when collapsing a track in the pattern view, then it will also be collapsed in the mixer.
Often, this is not desired.
For instance, with the new routing, one might have a group drum track that holds the following tracks:
The “Notes” track, as the name implies, only holds the notes to be played, while the audio is routed to the other tracks.
In the pattern editor one only wants to see “Notes”, while in the mixer one wants to see everything but “Notes”.