This has come up before. There are overriding controls on certain aspects of how a sound is designed/controlled. It's most likely a design parameter that is overriding the basic volume controls. If you investigate into the [EDIT] menu you might discover something assigned to the "volume" parameter.

That had occurred to me. I looked around, but I wasn't able to figure anything out...

I looked around some more and figured out part of the puzzle. One of the Parts is set up so that Assignable Knob 6 controls the Part Volume. What I have yet to be able to figure out is how the Foot Control (which generates MIDI Expression events), is set up to control Assignable Knob 6.

Could be I just haven't found it yet. Could be I'm looking in the wrong place…

In the PERFORMANCE "Solo Cello AF1" the Volume of each PART is under direct parameter control of the respective PART's Assign Knob 6.

Basically, the Performance is two PARTS.
PART 1 is the Solo Cello1 instrument, while PART 2 functions as the Solo Cello2.

You can balance the mix between the solo cellos with the Super Knob. By the time it reaches 12 o'clock both PARTS are playing together, if you continue to turn you are fading out the PART 1 (Solo Cello1) and fading in the PART 2 Solo Cello2.

Exploring the assignments here you will discover that on the upper "COMMON/Audio" level of editing the following links to the Super Knob are made: Press [EDIT] > Press [COMMON] > Touch "Control" > touch "Control Assign" > activate the AUTO SELECT function touching to turn it green > Now you can turn each Assignable Knob in turn to have the DISPLAY FILTER show you what is assigned to each of the Knobs 1-8 on this level.

The first knob, upper level, ASSIGN KNOB 1 is linked to control PART 1's Assign Knob 6

The second knob ASSIGN KNOB 2 is linked to control PART 2's Assign Knob 6

This means that whatever is assigned to Assign Knob 6 of these two PARTS is linked to the Knobs under Super Knob Control... and will move accordingly.

You can navigate to the Control Assign screen and view these assignments: [EDIT] > [PART SELECT 1]... lower Common > touch "Mod/Control" > "Control Assign" > AUTO SELECT and move Assign Knob 6:

Select [PART SELECT 2] > move Assign Knob 6:

This means that when you move the Super Knob - the behavior of the PART 1 parameter linked to its Assign Knob 6 will mimic the behavior of the Assign Knob 1 on the upper Common/Audio row. You can see this from the "Solo Cello AF1" HOME screen. As you turn the Super Knob from minimum toward maximum, the Assign Knob 1 reduces from 127 toward 0. You can see this movement by going to the following screen:

From the PERFORMANCE (HOME) screen
Touch "Motion Control" > "Super Knob":
See how AssignKnob 1 is set so it is opposite: Value 1 is 127, Value 2 is 0. Meaning any PART's Knob linked to this particular Knob will move in reverse... reducing values as you turn up the Super Knob.

Also from the HOME screen you will notice that the behavior of Assign Knob 2 is to go from 0 toward some value slightly more than half-way up... (it is actually from 0 to 89. This controls the PART 2 volume. You can also see this on the SUPER KNOB screen

On the main HOME screen the value you see for the FADER ("90" for each of the two PARTs) is the original Main Volume (cc007) setting... and represents the maximum toe-down volume when you move the Foot Controller 1 which sends cc011 Expression. Expression always works *within* the maximum set by the Main (channel) Volume cc007.

When you turn the Faders down to 0, you may notice that the sound does not completely disappear because of the settings within the PART Control assignment. "Bipolar" as the Polarity setting means we can move above and below the currently stored setting. (Programmable "Depth", if you will). The programmer here was not interested in silence at the minimum because you can use the FC1 (Expression) pedal to control that during performing.

Both Cello sounds have the Assign Switch 1 switching from POLY to MONO so you can articulate a legato phrase properly. When the Assign Switch is engaged and you play legato on the keyboard, the Part sounding will switch to the Element with the OFFSET sample, the one without the attack portion (bow stroke attack) of the waveform.

Hope that helps.