Roots stopping autoattacking

I've noticed in a few bot games recently that when I've been rooted but still in autoattack (AA) range I'll still just stand there making the wind-up motion but the hit never follows. Most recently this happened while playing as Alistar when I was rooted by Amumu ult while still standing next to Amumu and Darius. My E was activated before the Root went off (which shouldn't matter anyway) and despite being in AA range, my attack wouldn't finish so the stun wouldn't land. And for those wondering if I was perhaps just outside AA range, I wasn't. I was attacking Darius and we were quite literally next to each other. I've noticed this playing as Ryze or Lucian against Lux and Morgana, but just attributed it to me being barely out of AA range. Since Roots don't stop spellcasting or attacking, I'm not sure why this would be happening.