Woespike does not extend the strongest impale

Playing what seems like the obvious build for this item (crushing fist for a big impale, cyclone to keep extending it), and I can clearly visually see the health bar slow 1s after crushing fist, which at my attack speed is when i would burn through its 7 impale stacks.

Note that with my setup (perfect 72% extend woespike + 10% extend mastery), it should take around 5 times that long for the impale to expire from hits.

Testing in pvp with 100% extend (double woespike) shows the same thing.

Since I have 50% increased impale duration from harpooner, it is not just a product of having short impales.