While in "Live set mode", with the switch "Part/Element" set on "Part", i've noticed that the 1-4/5-8 switch does'nt work: it does'nt change anything. Back in Performance mode, the switch works correctly again.

It maybe just semantics but, there is only Performance Mode. Technically speaking, when you are on the LIVE SET grid screen, you are in Performance mode.

When you make a selection on the LIVE SET grid screen, you are selecting a Performance (from among 16 per Page). This Performance appears elsewhere in your MODX. The LIVE SET is simply an organized list for recall. (256 steps - where each step is a Performance, or a Song/Pattern/Chain linked to a Performance).

If you were to press [EXIT] or [HOME] you would move from the LIVE SET grid screen to the current selection’s Performance (HOME) screen, Song screen, or Pattern screen. But in all cases you are in the Performance Mode.

Whether or not what you observe is “defect” or not, I can’t say. But is likely related to the SSS function which requires your KBD CTRL active sounds be located exclusively in the first four Parts, 1-4.

If you need to address the Volume Sliders of Parts 5-16, it is recommended that you press [EXIT] or [HOME] and do so from the screens that give you access to any of the sixteen Parts.

Extra Credit
Please see the following article — “Performance Basics 2 and Live Set” specifically, the paragraph on “New Concepts: LOOKING AT THE LIVE SET GRID SCREEN, OR NOT“