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Hmm. That's pretty interesting. I did discover that rolling back to 20.04 with Pipewire created the same problem and then completely removing Pipewire (all packages) resolved the problem. I am now upgrading back to 21.10 minus any Pipewire modules to see if it works.

But... assuming that the problem is as you say, would the simplest solution be to add an input from a muted mic (or a null sound card device, like a muted hdmi output) to the bus input? Obviously, connections via jack routing are very logical and direct. It would be nice to keep that setup, even if it means using a simple hack.

EDIT:

I can now confirm that Pipewire causes the problem and the problem is the Jack "zero-copy" limitation. I also found that routing busses through an external Carla environment (flat EQ modules or something similar) corrects it (as suggested in the threads that you linked). I also found that routing busses through spare card slots does the same without having to go through an additional application. In my case, I have 3 HDMI slots showing, which is just enough to allow a mixdown through 2 bus layers between the initial bus and then the master. Probably the best solution is to create a simple Carla session to accompany each project... loading the project first, disconnecting all default connections and then loading the Carla session (apart from using a session manager of some type).

So, in the end, I am back on Pipewire and in 21.10 (where I wanted to be).
Thanks so much for your help and direction in sorting this out. I have been fortunate to always have a hardware/software combination that didn't reveal this issue over all these years that I have used Qtractor!

Take care!