This is a good observation that was brought to my attention by a client hooking up the cameras this way. When I tested in house, I found that when using RS-485, any camera after the first will zoom all the way to its limit when the zoom button is pressed once. This makes it impossible to use the joystick for zoom control.
What you have to do, is go into the on screen display of each of your PTZoptics cameras, and go the setup or communications setup menu, and change the protocol from VISCA or PELCO to AUTO. Once you do this, set the V address, P_address, and the D_address to the address of the camera (first camera is one, second is 2, etc). Once you do this for all of your cameras, it should get rid of the zooming issue you were experiencing.