MyDroneAI's method IsValidTarget() is bugged.  It checks this to determine whether a target is still valid:
(target is MyCubeBlock && ((MyCubeBlock)target).IsFunctional)

However it fails to also checked whether the target is "MarkedForClose".  The result is the drone will continue to follow the target but its Position is now 0,0,0.  IsFunctional() still reports true.  Perhaps, the bug is IsFunctional should check MarkedForClose instead?