Well I played with this quite often lately, and I noticed its stacking is pretty weird.
Sometimes when I'm going to run out of the buff and I'm trying to maintain it by attacking minions or something, it feels like it has a pretty big (~0.5sec?) register time for the stacking to take place.. So I'm losing the buff a ton of times even though I just autoattacked or entered combat :|
Is it working as intended? I think the stacking registeration should be instant... >.<