QS8 Synth- effect transferred to other sounds on split keyboard

I have a question about the QS8.1 synth:

When splitting the keyboard, sound #1 has a vibrato effect. When this sound is playing in one range of the keyboard, sound #2 (which is playing simultaneously in a different range) also receives its vibrato effect. When playing alone, Sound #2 does not contain vibrato.

Is there any way to remove this effect from the other sounds on a split keyboard?

Thank you,

*Edit: This may be caused by aftertouch? And I actually am able to produce the effect when Sound #2 plays alone.
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