Cf. below, Dingle--Michelson Morley experiment. Case of receding mirrors.
--------------------------------------------------------------------
For the case (below) of mirrors receding uniformly away from the lamp, it is a certainty, that, while we are stationary with the lamp, both rays will make it back to the lamp simultaneously.
Next we journey to the right and look at the happenings at the lamp. Once again (obviously) the light makes it back to the lamp simultaneously.
Now we project "real motion" into the system, by saying we are "stationary" with the man moving to the right. We see the apparatus as a whole journey to the left.
Now we witness the top ray bending as it navigates its path to the left and up to the receding mirror, and back down again to arrive simultaneously at the lamp.
If you deny it, you will have to explain how this top ray navigates to the left, and upward, to the receding mirror, without bending, in its effort to get back to the lamp simultaneously with the the other ray, as it must.
You will also have to demonstrate how it is to do this, without bending "in flight".
And it can only bend "in flight" if it knows it is headed to an absolute event that lies in the future, and for which it must reach (at all costs,) in order for the reality to be consistent among the observers.