The Master Volume Fader in Omnisphere, Trilian, and Keyscape has a lower resolution than the Part Level Faders (fewer increments) or Expression, so quick changes when controlling it with automation, MIDI CC, or a volume pedal can cause the 'zipper effect." You'll find the Master Volume Fader in the top right of the instrument.
Please use the independent Part Level Faders instead, as they have a much smoother response. You can find these in the bottom left of the Main Page (as shown above), or in the Multi Page as shown below:
For Keyscape, make sure Expression is turned ON in the System window (as shown below), then automate volume using MIDI CC#11:
