Vateshran's 2H weapon states that while Momentum is active, using stamina abilities grants you a stack of Frenzied Momentum. After 5 stacks, your next Heavy Attack detonates the stacks for aoe damage.
Two things go wrong when using this:
1) If you swap to another weapon and use a heavy attack with 5 stacks, the stacks are consumed and do not detonate for damage.
2) If momentum wears off and you use a heavy attack with 5 stacks, the stacks are consumed and do not detonate for damage.
Now, I can get it NOT detonating if you're not actively using the vateshran 2h weapon or if momentum wears off. But to have it waste these stacks can't be intentional. At least I sure hope it isn't...