Once is not constant
Also, that happened in some book hardly anyone has read or has even heard about. The Clone Wars in comparison is much closer to the source of canon from GL since he's involved in its making personally... and as silly as Maul returning may be, they're not going to do something like Luke and Leia travelling back in time. People seem to like to blow things out of proportion.
We don't even know in what way Maul is returning in TCW yet, it could just be some kind of holo-recording he left in case of his death that instructs whoever finds it of something, yada, yada.
We'll see how it all plays out when the episode or episodes in question actually air.