I've been noticing this recently on my matches at Vayne and Jhin.
For Vayne, Tumbling and attack moving on a target that's about to die. If you get the last hit it's okay, but there's also a window to me it seems, where it's possible you'll get the last hit before a caster minions missle hits it. But if the caster last hits it your attack animation happens the projectile fires off. But the projectile fizzles, your attack animation cancels and primes a new attack on an existing unit. Afterwards there's this massive lag.
Same happens to Jhin, especially when 4th is primed, attacking moving after using Q gets dodgey. Again this is when the case where the minion or target you're intentionally attacking moving near dies. The animation happens, cancels, projectile fizzles, and attacks nearest enemy unit. I'm sure that's part of the code but I've ran into it maybe twice now.
As an additional note I do use smart cast and the stutter seems to happen after I cast an ability.