First, check out this scratch

When I use _notify () function on 2D Array Tracer with 3rd argument (v) as 0, the value in tracer is not updated even though 0 is a perfectly valid value.

This is because line 133 of array2d.js says:

if (step.v) {...}

This disqualifies 0, and doesn't allow us to set it as any item's value. We need to dis-allow all false-equivalent values apart from 0.
I think we should even allow null, but I haven't fixed for that, only for 0.

The result is as expected on my ghpages