It is possible it was wrapped too long and accumulated too much moisture or that the bed was not raised enough, or both - I lost a musella lasiocarpa this past winter that rotted out (it looked fine from the outside) and it was not only in a raised bed but a terraced one, and yet a second lasiocarpa only about 3ft away is still doing great.
Perhaps it was damaged prior to wrapping and that made it more susceptible to excess moisture........anything is possible
I'm sorry for your loss
