Even if you know these characters are coming back, they still died, in a very sad way.

That's not dying though... What makes death a sad moment (for most people) is the finality of losing someone's entire character at an unknown point in time. If we all died for 5 days when we hit 30 and then came back to life, nobody would be bawling their eyes out over it. If we were all immortal until we hit 80, and then we all died, the way we perceive life and death would also be entirely different.

It's one thing to write a sad scene. Any idiot can slap together some cliche's and write something that's sad on paper. It's an entirely different feat to get the reader to emphasize with the dying character on a level that goes beyond "that's so sad, Alexa play despacito". Knowing a character isn't really dying makes that rather difficult. Clearly you felt swayed by it, but that doesn't take away from it being a very poor writing/marketing decision.