I have used Fusion for many months now with my CNC router, and I discovered very early on that the G28 operation is complex and confusing if you are using Mach3. The answer however is simple - just turn it off in the CAM options. All works as expected then! I read this somewhere on a forum, along with long discussions as to what it should do, and what it actually does. Dazed and confused, I just turned it off, and I haven't looked back.

Hope that helps
Graeme