If automation is trim+read style, then the envelope does not change when changing fader. Movement of fader trims the envelope of the same amount everywhere.
So if fader moves after envelope is made it might not correspond.
Very clever actually compared to absolute envelope(Read) where you have to change the entire envelope if needing to trim anything all over.
It's reasy to change automation style if you need. Rightclick automation symbol on track panel.
And to be able to have separate volume handling(CC07), I think instrument need to receive on different midi channels(or two different instances of sfz). If trying to do it on the same channel on the same instance will not be the result as you expect.
But don't forget that volume is the sum of volume(CC07)+expression(CC11)+velocity(not arithmetic sum but an algorithm with those parameters).