Anivia's E double damage bug

So I've been noticing lately that sometimes, when I cast me E on people that I've slowed with R, it doesn't always deal double damage. Case in point, my most recent game I tagged an Ahri with my e after casting r on her, she dashed out of R, was slowed, and my e connected, and it dealt normal damage. I can tell the difference almost immediately because it deals a LOT more damage when doubled up. I was able to connect it some times but other times it would bug out and not deal the double damage. Is this a known bug? I had it happen to me several times during the game. I have had it happen multiple games mainly with champions that have dashes, where the slow procs, and I cast my e, but it still doesn't deal double damage. THIS IS NOT AN EVERYTIME BUG, just maybe 30% of the time, but it does happen, and it really doesn't feel good for sure.