jit.expr: it doesn't get much simpler than this... so what am I missing?

I've come across some very strange behavior in the jit.expr object. Take a look at this patch.

Click the button at the top and look at the values displayed in the jit.cellblock object. When I do this I see all 2's.

Now click the button again. Now I see all 3's. Why is that?

From what I understand about the order events are sent out the "2" message should arrive at the right matrix first. Then that matrix should send its output to the expr. But since it enters the right inlet nothing happens. Then the "1" message goes to the right matrix and it's output enters the left inlet, triggering the calculation in the jit.expr object resulting in all 3's in the jit.cellblock object. But that is not what I am seeing.

What is going on here? Is this a bug?