Tactical Camera if I understand your description of the symptoms?
I hardly ever play in tactical mode.
I sometimes switch to it when Solas decides he's a tank or Sera thinks the best arrow shots are at point blank range. I just move them and then go back to regular mode.
Twin Fang is your backstab ability. Literally, it deals bonus damage from behind. Deathblow is exactly that, a death blow for securing kills. The enhanced Deathblow hammers that in by resetting the cooldown on successful kills. Also, I don't think Deathblow has any damage bonus with backstabs, where as Twin Fangs deals 2x damage (I think?) from behind.
It should still work the same way when it comes to using the ability.
You click the power, your character moves to the enemy (if necessarily), and it fires. Yet Deathblow frequently fails to fire or my PC attempts to move to the enemy by shuffling around and sometimes even walking away from my target.
I tried playing with a 2-handed warrior today. Mighty blow works correctly 100% of the time. I click, it fires. If I don't hit anything, there is no cool down.