Inconsistent hopper flow

On the first 2 screenshots : the circuit

On the last :
1) Expected output in the chest when you press the button
2) Output obtained after pressing the button
3) Output obtained after pressing the button after a logout/login. Which is incorrect !

I think it comes from hoppers that refresh too fast or too slow compared to the others. This behavior happen often and is very random (seems to depend of the XYZ position of the hopper).