This is not problematic in pre-Einsteinian physics, where arbitrarily fast signals could be
used in place of light. As faster and faster signals are used, the interval between departure
and return shrinks toward a unique instant, which then must also be the time that is
simultaneous with the arrival of the infinitely fast signal at B. In SR, however, nothing can
travel faster than light.
2
In fact, if standard simultaneity is adopted in every inertial frame
of reference, then it follows in SR that every instant in the open interval between the
departure of the light ray from A and its return is simultaneous with its arrival at B in some
inertial frame (where A and B still refer to fixed spatial locations in the original frame).