Looks like it happens when your Q and AA hit the target at the same time. It happend at least 6 times this match, so it is a frequent bug.
Reproduce: Q a minion and AA it at the same time. Sorry for the audio, no idea how to use Sony Vegas.
Edit1: Don't know how not to show other people name here, I apologise for it.
This is really frequently happening bug. In my last match I could reproduce 10 times during 6 minutes. It works as the OP says, if you happen to AA the minion/person which the Q is bouncing from/to, it'll disappear. Most frequently this happens after the 2nd bounce of Q. This bug is really annoying, while you're trying to trade with enemy laner or trying to clear minions and you see that your Q doesn't do dmg and it completely disappears. This started happening after the patch 6.6.
