thanks for the feedback. There is a bug with z.
However, the z position is intended for something else. A sort of "3d", for example if a character jumps, server side, it's the z position that changes and not y.
client side, z will change the (x,y) positions for rendering and not the (x,y) for logic.
In short, z position is different from z index of sprite
I'll see how to put this posibility 🙂