Trying to reproduce this yet -AGAIN- but **** it's annoying not being able to get the stun in when you want it to just because of that invisible FF I'll post my findings.
Managed to reproduce it a couple times so far... Rank 3 flash frost, about 45 minutes in, 3/4 mana pool and about 200 health. Just spammed it with basic attacks so far. it disappeared about half-way to the targeted location.
Been about 90 minutes, getting tired of this so here's what I've got so far. five occurances of Flash Frost disappearing. Most of the time about half way to the designated location. Also noted that it seems to occur less than half a second after you attack. I've been able to recreate it three times but my timing is always 100% on.
Another bug to mention while we're on Anivia... With near endless mana regen (Chalice+golem+5archangels) you can keep glacial storm up nearly forever. However, the little green outline and it's animation just stop playing after a while but you can hear the noise and see the enemy get slowed.
I'm still not sure if this is really the cause but it's as close as I can get for now.