To be as concise as possible, I think the issue you're running into is that the game basically calculates whether you've hit or not the moment you activate the power, so even if you've missed, you still have to go through the animation for the power. On top of that, if you're performing an assassin strike from a hidden state, there's a period of time where you can interrupt the power, so even just tapping a movement key can kind of cancel the power. Lastly, it is possible you're fighting an enemy with a high perception that sees through your stealth, or you're missing, which could break your hidden status.