Eviscerate Cooldown Bug

I am combining Measured Retaliation (50%), No Forgiveness (20%), and Runegraft of Time (20%) for a total 90% chance for Eviscerate to remain usable but the skill very often goes into the unusable state after just 2 or 3 uses.

My Eviscerate also remains usable for 6.8 seconds after blocking so it's definitely not going on cooldown because of it timing out.