
Thanks, Lars.
I can confirm that I can reproduce that problem using your steps, and it does indeed appear to be a bug. Buffers for audio mixer buses are intentionally not allocated unless something is routed to them, so as to save memory, and it appears that the buffers for a bus may not be being identified as needed in your test case, which would result in no sound being heard from that bus. However, as long as the bus is a member of any group at all, even if nothing is routed to that group, then its buffers are guaranteed to be allocated, so it will work properly.
Hence as a work-around for now, if you want to route ranks directly to any mixer buses (instead of going via groups), please ensure that those buses are also members of some dummy group (e.g. group 1, or even group 1024).
My apologies for the bug.