When you say pressing [PERFORMANCE] (HOME) fixes this, this seems to indicate that you have a Part selected.  When a Part is selected, the sliders are adjusting the element levels.

Although, this should be reflected in the LEDs that tell you the slider mode and I believe you're saying the LED illuminated is indicating Part levels and not element/operator levels.

I don't see anything unique in the Rd 2 Gallery in terms of how the sliders would work (like some organ Performances that automatically place the sliders in element/op mode).

One wrinkle in my ability to directly help out is that I own a Montage so I don't have this 1-4/5-8 feature since all 8 sliders are there already without a need to bank.

I guess if you suspect something fishy - maybe check your firmware version is up to date.  It may be already - but you didn't mention anything about it in your message. 

Use 'Contact Us' link at page bottom for support. They can tell you if they suspect a hardware/software issue.

Your instrument could have a button, or other, issue. As far as I know those lights should work properly from the category search screen. But I have no way to test that myself.

I've seen other posts here where people had problem with volume sliders which turned out to be caused by assignable knobs or other special settings for that performance

That is likely the case for volume with part 5, at least on my Montage M8X. Here is what I see

1. Keyboard control is OFF for part 5 except in scene 4.

2. Scene 1 is selected when I load the performance and that means keyboard control for part 5 is OFF.

3. Keyboard control OFF means there is no sound produced by part 5 when I play the keys UNLESS I have part 5 selected.

4. The insert effects for that part have control assignments for both assign switch one and some knobs. One of those knobs is also being controlled by the Super Knob

NONE of the above should, but could I suppose, affect how the part range lights work. Which is why you need to contact support.

The firmware says "Version 1.00.0   (c)2022", there doesn't seems to be any newer version available for the MODX+.
Is there someone with a MODX+ which can confirm that the 1-4/5-8 button for the sliders is locked to 1-4 when the sliders are in Part mode on the Category screen?

You may want to upgrade the firmware.  Not that this will necessarily fix anything but it is a good thing to do (and may fix something).

https://usa.yamaha.com/products/music_production/synthesizers/montagem/downloads.html

That's for the USA.  Go to your similar localized page if needed.

There are two versions of the firmware.   One is a large image that upgrades from 1.x and this is the one you should pick.   One upgrades from 2.x and this isn't the one for you.

Correct:

MONTAGE M OS Updater V2.10 from V1.x (4GB)

I'd delete my previous message if I could.  It doesn't apply at all to you.  I've got to be more careful in today's faulty forum landscape.