Herald of Thunder Toggles Off Randomly

Ever since I started using the Herald of Thunder skill, I have seen the skill deactivate randomly without me touching hotkeys or messing with it in any way. That automatically reduces my lightning damage by 20% when I'm too busy to notice the missing icon on the top of the screen. This has caused me to die at times, and it needs to be fixed. It's still taking the reserved mana, but just shuts off whenever it wants for some reason.

To make certain, I changed the hotkey for Herald of Thunder to Ctrl+T to make it nearly impossible for me to accidentally turn it off in the heat of battle, but it's still happening.