Hi Philippe:
The simplest way of reproducing what I'm hearing is
New -> Simple Program
Then, while holding down a key on the keyboard, press the On/Off button for FX1, then FX2, then FX3. The volume gets incrementally louder with each FX On/Off button pressed. The On/Off buttons do not light up, and the volume stays there, even if I try pressing the On/Off buttons again.
This is not a big issue, but it may be related to the other volume issue(s) that have been raised.