I did some testing with magic arrow at max FC/FCR and it seems the spell is cancelled or something, it doesn't do any damage when I don't put any delay in my macro:
cast magic arrow
wait for target
last target
-> I had to insert a wait here, up to 400ms (where it still fails to do damage from time to time)
Considering the delay and the similarity with fireball (fire damage), I will stick to fireball as I don't have to add a wait and is more reliable (and does more damage). It's not like it was a big deal though, I can perfectly live with fireball.
I wasn't sure if it was a bug or not so I guess I'll just post it there anyway.
Have fun!