I have the same issue. I tried opening at least 20 flower packs in different conditions: during the day, night, as a ghost, during a festival, after restarting the game. It never worked.
Someone also wrote that you can buy the death flower from the mysterious merchant, but he never has one.
Ditto on not getting a death flower from opening starter flower seed packets in the cemetery.
You can get a death flower from the Mysterious Rancher if you have Horse Ranch and befriend him.
As a ghost, I was able to deliver a death flower without ever having one in my possession. As soon as I took the quest from the Death Obsessed Lenore Member, the Obtain a Death Flower step was ticked off. And no, I checked several times, and there was no death flower in my possession. Not sure if this is a bug or working as intended.
