Reproduction as seen in the video:
1. Have Vayne basic attack someone, not sure if she needs to R before it.
2. Have the targeted champion become untargetable i.e stasis before the attack hits.
Vayne's attack goes through the untargetable part and adds a stack of her W.
What should've happened:
Vayne's attack fizzles mid air, without adding a stack.
Not sure if this happens only on the final W stack, as seen in the video, or with all stacks. Also not sure if this only happens with stasis.