Yeah, I hate that shit as well. It's one of the reasons why I feel no satisfaction after beating a boss. I had so many experiences where they have pulled a "Somehow, Palpatine returned" that I just don't trust the happiness sequence that usually follows a bossfight.
You beat the boss! BUT WAIT!!! It's not actually dead!
And then you say "wow, no way!" ironically as you watch its health bar refill.
Who could ever have expected this?!