Physics Matt has a blog on the issues with FTL communication.
At one point they use an example of a relativistic observer seeing a FTL message arrive before it is sent, to show how causality is broken.
However, the destination that is receiving FTL message will already see the message before the inciting event happens according to them.
Why is the extra observer needed for this? They say:
Earth calls Proxima Centauri, then the light reaches Proxima Centauri. No problem: though the Proximal Centaurians hear about the event "early," no causality has been violated. ... Right?
They hear about it early because to them it hasn't happened yet.
The lines of constant time for the ship show they arrive at the destination after the message is received, but from their point of view on arrival the message hadn't been sent yet.
They show that you could, from the destination end send out a light speed message to say they got the FTL message. The Relativistic observer receives it before they arrive, but that is still after the initial event happens... causality preserved? Yes, the initial light hasn't reached yet, but that's exactly the same situation without the relativistic observer. We already lost simultaneity due to the distances involved (if the two static frames of reference were just good at co-coordinating (i.e. not relying on FTL messages) sending out a signal, it would looks like the destination sent theirs out early.)
How is this different to the case without the relativistic, third-point of view?