IT DIDNOT FIX THE PROBLEM
DRAW A BLOCK ABOVE Y=0. IF YOU ZOOM OUT AND CLICK MIRROR DISPLAY IT SHOWS BOTH BLOCKS. IF YOU ZOOM IN ON THE MIRRORED BLOCK, FAR ENOUGH TO TAKE THE SOLID BLOCK OUT OF THE PICTURE, THE MIRROR BLOCK WILL DISAPEAR. IN NX3 IT STAYED VISABLE NO MATTER WHAT.