Nautilus Passive Root-Auto Animation - not consistently playing, but still applies root

{Bug Description} Nautilus' auto-attacks utilize a special animation when they are applying the root from his passive ability. This animation did not consistently play, but roots were consistently applied, throughout the game.

I've uploaded a link to the full ARAM gameplay in which I noticed this bug.

The first instance of this bug occurring that I noticed is when I auto-attack Trundle at 3:00, killing him. Since it was my only attack against Trundle, it should have used the special animation - but did not.

The bug occurs again throughout the game, for example, at 8:28 immediately after hooking Trundle, the auto-attack roots him but does not play the special animation. At 9:30 I auto-attack Trundle as he dives our turret, and the special animation DOES play. It also doesn't only affect auto attacks sent to Trundle - perhaps the clearest example of the bug occurring is the auto-attack I hit Vladimir with at 6:44.