Just guessing: is the ZynAddSubFX plugin set up with “MIDI learn” such that the volume is mapped to a controller (e.g. the mod wheel or expression pedal on your keyboard / controller)? If so, then it might be jumping to whatever the current state is on the mapped controller,
No, “LEARN” in Zyn is not selected.
I would add that quite a few times the “instrument” (Bass, Piano, etc.) I had chosen in Zyn is also changed, replacing it with the basic default one.
All this even with “Restore Patch” deselected and with “MIDI Event Filter”+“Block Program Changes” active.
But it seems I’m the only one with this problem.
a.
I get you, man. It’s a bummer. I’ve had it happen with ZynAddSubFX, too. While we’re at it, the plugin state of another oft-used plugin, sfizz, tends to do the same, i.e. go in limbo/oblivion. If a virt instrument has tuning in sfizz, I don’t bother to set it until mixdown Peace.