I just probably messed a boolean in the engine properties but can’t find it, i’ll explain you exactly what happen.
First, I have a multicast that send the AI an anim montage to play.
In this anim montage I have 2 notifies theses 2 notifies start 2 function on the AI that I execute only if there are called on the server.
The first function register a starting point and the second the end point and then sphere cast between theses point and apply damage to all hit.
The thing is that when the server doesn’t have the AI in his viewport (I’m not using a dedicated server) all the notifies doesn’t get received. All the animation works well on the client side but the server doesn’t get any notifies, so the damage can’t be applied.
If any of you already had this issue I would really appreciate a little help,
Thanks and sorry for my english 
Edit:
I have digged a bit more in depth and it seems like in fact it only the trace that return 0 actor hit unless the server have the AI who trace in is viewport