Well, my long answer have been completely cut out after I refreshed the page and I cannot edit it…
So, in short, your calculus for the End of the Line trace seems a little weird. You are telling : “give me the position of the PlayerCamera + forward * 5000”, but I believe you want to check if the Slender is watching the player, not the opposite.
Also, you do not need a line trace here since you are using the OnSeePawn and at this time, you already know that your player is being seen (after checking that the seen Pawn is the player).
For future operations with the camera transform, you should use PlayerCameraManager(0)->GetCameraLocation instead of GetWorldLocation because your PlayerCameraManager might not been following the camera (not sure, but I believe).
Finally, although I am French, it is better to continue in English since someone else can take the current conversation and help you, or for another one that look for an answer on this same issue.