Fiddle Sticks Fear vs Lux Ulty

During play in ARAM, playing as Fiddle Sticks, I used his fear on Lux shortly after the beginning of her ultimate animation. The fear proct'd above her head during the animation, however, her ultimate was not interrupted at all by the fear. Rather, her animation continued as normal, her ultimate went off, and then Fiddle's fear timer (what was left of it) finished off with Lux moving away slowly. Again, the fear only functioned after her ultimate completed, and then the fear lasted for the remaining duration. Isn't Fiddle's fear supposed to take some precedence?

Thank you!