Hi,
Unfortunately, mumax-view was designed to view magnetizations (a vector quantity), not a scalar quantity like geometry. If you try to plot geometry, it will do this repeat issue (see e.g. this :
previous discussion). I don't know an easy way to get a nice 3-D picture unless you import it into something like Paraview, which is a bit of work.
If you are just using it for your own verification and don't need the picture to be pretty, a cheap trick you can do is to just set the magnetization m=(1,0,0), and plot the magnetization. It will be (0,0,0) outside of the geometry. Otherwise you probably will have to stick to things like the built-in mumax3 GUI which gives a view in slices through the z-axis
Best,
Josh L.