Mixer showing not more than 16 channels

I have 2 instances of Halion Sonic SE. The mixer shows only the first instance.

That’s odd: you should certainly see two sets of outputs, one after the other. There’s no disambiguation between the different instances of HSSE, so possibly the issue is just that you can’t tell the outputs from each instance apart?

I added a third instance and it showed up in the mixer. I deleted the second. All’s well.