As you have concluded - what are the odds? Here's what's going on...

The VOICE called "Sixpack" (also available in the MOXF, Motif XF, and Montage) is one designed specifically for use in a HYBRID (Velocity Zoned) Arpeggio situation. Let me explain... the reinvention of the Arpeggiator by Yamaha includes several different Key Modes and different ARP types - these go far beyond your traditional up/down, down/up and random Arps traditional found in Arpeggiators.

Arps respond to note commands that are defined by Key Range and Velocity Range.

The HYBRID ARP pattern is one in the Velocity Zone category... Velocity 1-111 will trigger a sound and play normally in response to Key-On messages, and Velocities 112-127 will be provided by the ARP phrase. So in a Layered situation you could place a Pad or String-type sound to play normally when notes are 1-111, while the ARP phrase triggers the sound velocity limited to note-on events that exceed 112. "Sixpack" is such a sound

This is why when you turn the ARP Off, the Sixpack sound only responds to velocities higher than 112. This Voice is designed to play in response to velocities 112-127 only

You will find this Voice Layered with a sound responsible for playing velocities 1-111 as in Performance 127... "Cascade". The Layered Part plays in response to your normal note-ons, but Sixpack only responds to 112-127 (provided by the Hybrid ARP phrase.

This version of Sixpack was especially edited for use in an MX Performance. You don't get to get to the Element level of editing on-board the MX (as on the MOXF, Motif XF, or Montage) without the third party Editor (with the Melas MX Tools, you could get in and reprogram the Voice).

But bottomline is your MX, and the one you returned, are absolutely behaving as they should.

Extra Credit
If you are into getting deeper into sound creation with your MX, I highly recommend the John Melas Editors for the MX- they are awesome and let you get to all the "hidden" parameters! Well worth it for those who enjoy editing and sound design.

Element 5 and 6, are brought in by the ASSIGNABLE FUNCTION buttons (MOXF, Motif XF, Montage) these switches are not found on the MX. The [AF1]/[AF2] buttons control XA CONTROL - Expanded Articulation Control, which allows the performer to bring Elements on demand. This feature is not available on the MX.

In the MX version of "Sixpack" these Elements are set to "Normal", so they are set to play, but their Volume is set at 0. With the Melas Voice Editor (MX) you can bring them in.

They are also addressable via Sysex:

Element 5: F0 43 10 7F 17 41 04 1A dd F7
Element6: F0 43 10 7F 17 41 05 1A dd F7

where "dd" is the volume value 00-7F

If you send these to messages with a volume value, you hear these two (hidden) "Pad" Elements. Then STORE your custom version of Sixpack to one of the 128 User locations on board the MX.