Suppose that we have two platforms: A and B. Each of them has a special photon detector on them. Detectors are constructed in such a way so that only photons falling on them with 90 degrees incidence angle will be detected.
Now at $t_A=t_B=0$, the two platforms are parallel to each other with sensors facing one another(as shown on the sketch) with platform A not moving in the lab's reference frame and platfrom B has velocity of magnitude $v$ towards $+x$ direction.
At $t_A = 0$, a flash of spherical light is ejected from the point situated between sensors and equidistant to them.
My question is, what which sensor is going to detect light on them? Sensor A? Sensor B? Both of them? It seems natural to me that only sensor A detects light, however doesn't a photon reach sensor B perpendicularly in B's reference frame as well?