Observing properties from control surface object ...

hi there,
for a project, I coded a very basic python remote script providing values of each knobs of an hardware controller.

It works fine using max objects (I mean live.observer and .object etc)

I used to code JS stuff when I need to observe a lot of parameters.
Indeed, using JS arrays filled with objects set up with a property to observe and callbacks is really elegant and efficient too.

But I'm stuck in my case of Control Surface stuff.

At first, I thought I had to provide something more in the Python code, but my [live.observer]s works very fine while observing the property ... value. So this is something else.

The JS complains about "property value doesn't exist"

I surely missed something.
Anyone here?