Runeglaive Enchant sometimes skips an autoattack after ability

While I was playtesting the new runeglaive enchantment while playing Vel'koz, I was attacking the mid inhibitor turret when I used an ability, and it canceled the first autoattack (the one with sheen proc), and had to wait for the second autoattack (as if the first had gone through. Second autoattack did proc sheen). It happened twice, but I have been unable to replicate it afterwards. Was it a visual issue or did it actually skip the autoattack is what I am uncertain of.