I've had a few times where Legion's power immediately cancelled itself after I started using it. And this issue only happens immediately after I start using their power.
Were you rapid tapping your button to activate it ahead of time to get it the split second it comes off cooldown? The same key that activates it deactivates it. Otherwise, I would double check your keyboard and mouse keys aren't sticking as this doesn't seem to be an issue with the patch.
I'm just clicking once, and I used to play Legion before SBMM, and it didn't happen back then.