The overall CG shifts back, but the CG of the majority of the mass that is in the fuselage in the form of batteries and payload doesn't move. It remains at roughly 25% chord and that is the load being carried at the swing/tilt hinge. The moment applied by the motors at relative to that 25% chord has to be zero to keep the aircraft in balance.
During hover the rear position motors are each carrying only their own weight and whatever a roughly quarter span of the wing weighs. If a forward position motor is lost, the other motors have no thrust solution and the aircraft will no longer balance. The overall CG is back a small amount but it looks to me that it lies outside the thrust triangle
Maybe I missed the easy to perform demo of an engine/motor out condition?
As long as they stick to electric motors and small aircraft they can get away with it. But moving to turboprops or turbojets to get far larger aircraft will not see a good solution to this problem.