After much hair pulling, Ive confirmed a bug…
I can select the staff of the instrument in write mode, press P, and get playback of just the synth. Open the mixer, and find that I can mute the only channel playing, and disable its Send, and still have signal hitting Stereo Out. The signal ceases to meter on the channel I muted, but Stereo Out remains.
Furthermore, I can apply a Filter insert FX on the channel and sweep Cutoff Freq, and get no change in audio, as in Analyzers on Stereo Out show no filtering applied, while the filter insert shows correctly in its FFT. Also, the affected channel doesnt seem to respond to its own fader.
It sounds like there is a mismatch between the output from your VST plug-in that Dorico thinks is being used and the channel that is really being used. Dorico can’t actually read any information about the channel utilisation from the plug-in, so it assumes that e.g. if you feed MIDI into channel 1, the signal will be returned on output 1. Perhaps your plug-in is returning the signal on a different output than the one Dorico expects?
However, I see part of my confusion was due to inconsistent behavior.
If I use P the problem occurs as described, but if I use spacebar, goto Mixer and solo, it is correct and FX insert filters and everything is as expected.
That is why I was frustrated during troubleshooting, and during the workflow ~ using space/P was giving different results
Thanks to you both ~ you both helped! Unused shows Surge XT has its channel, and also outputting 2 more stereo channels Scene A and Scene B =/