What is/are the PART(s) that are layered with the piano?

Piano is in the percussion family of musical instruments... percussion being described as instruments where the vibration is initiated by hammering, striking, or plucking the instrument. They all share the natural behavior that the sound will die out, due to friction, gravity and natural physical forces that work against the applied energy.

If you select a sound from the other family of instrument sounds (those that are self-oscillating - meaning as long as you apply pressure to the instrument, by bowing or blowing, it will continue to vibrate and make a musical tone) your results will be different. The behavior of the loudness shape of these instruments is what we need to discuss.

What you need to know:
The AEG (or Amplitude Envelope Generator), is responsible for describing the loudness shape of the synth sound. Yamaha AWM2 Synth Envelopes use a series of TIME and LEVEL parameters to describe the shape. You control the Level (Amplitude) and how much time it takes to advance to the next Level Point... this is also a kind of ADSR (Attack-Decay-Sustain-Release) with a bit more precise control.

When “Decay2Level” = 0 you are describing a percussion Envelope. If “Decay2Level” does not reach 0 the sound will sustain at that level until Key-Off.

The sustain pedal in a synth takes the place of you physically holding the key. Percussive sounds eventually die out, even if the key is or sustain pedal is held. Self oscillating sounds continue to sound as long as the key or sustain pedal is held. A person bowing a string instrument can apply endless pressure by altering direction of the bow. A person blowing a horn can circular breathe and apply endless pressure to the instrument maintaining a tone...

Whatever you want to behave like a percussion instrument will need to be edited so that “Decay 2 Level” is 0.
If “Decay 2 Level” is not 0 that sound will continue until you stop applying “pressure” (key-Off) - at Key Off, the “Release” (time) parameter describes how quick the Level returns to 0.

For more on the basics of Envelopes see this article:
“Learning about Envelopes”

For specific help in editing a MONTAGE Part, you will need to identify the Part... editing AEG can be done surgically on a per Element/Operator level, if desired. This is the power of a synth... you can do overall shaping or detailed shaping... and to get the most out of any reply would require it get fairly specific. Studying what has been done by the original programmer becomes necessary to get the most out of any editing.