The Voice location is stored in the Pattern Phrase header.

Translation: the Bank and Program location are stored in a special Phrase header (a set of information that is recalled whenever you recall the Pattern). For example, if you have USER 1, 001 as the location stored in Part 1, that information becomes a part of any MIDI data you record on Track 1.

The Phrase header stores what Voice location, what volume, what pan position, and if the Insert Effect is active for the Part. It does this when the track is placed in Record.

The only things that can change ''tis is if you send a command or commands that override this.
You can review and change what is contained in each track's Phrase header as follows:
Press the [REC] button but DO NOT press play, this enters Record Standby
Press [F2] VOICE
You can now view what is memorized by the Phrase header, you can change it as you desire to update this data.
Press [STOP] to document your new selection.

If the Parts still insist on reverting to pianos, then you or the original author of the tracks has Input Bank Select/Program Change info in the tracks or a GM Reset message is overriding your selections.

Let us know.

Okay, I've updated the parts as indicated and they are still reverting to pianos. I'm not sure how to check the GM reset or Bank Select/Program Change.

Thanks for the reply.

From the Pattern main screen, select Track [1]
Press [EDIT]
You will now be looking at the data in Track 1
You can use the cursor arrows to move through the list of events.
For example, 001:1-000 is the measure 001, beat 1, and clock tick 000 of the event
If you are the original author of the data there should/could not be a GM RESET command (F0 0E 0F 09 01 F7)... so it must be an inserted PC (program change) Event.
If it is a GM RESET, highlight it and DELETE it using the button s along the bottom of the screen.

Let us know what you see...

I don't see any inserted GM RESET - as described. I do see the area with note, gate, velocity though - as mentioned.

Do you own a MOX or a MOXF?

I own the MOX (8). I ended up restoring the factory settings and the patterns are now saving/playing back properly.