All observers are equally valid under SR; it would be very arbitrary to say that some of them are seeing "artifacts". Physics is based on observation, and the natural way to interpret our observations is to say that what we saw really happened. (Obviously this isn't true if some observable effect is distorting our perception, but that's not what's happening here - the observer who sees the person arrive before they left is a perfectly normal observer using perfectly normal equipment). Now if you want to say physics works differently for observers in different frames, that's fine, but it goes against relativity and all known physics.

Now assuming we accept that FTL-traveller really did arrive before they left, if that's all they do it's a matter of language as to whether they have violated causality by so doing. But it would take some very perverse physics (again, basically discarding relativity) to say they couldn't simply jump back, again going backwards in time, and arrive back at their starting point before they left it, at which you definitely have causality violation.