Sorry for your loss. :hugs
Similar occurrences and a long time memory has been discussed before. Some people find it hard to believe a chickens remembers persons (rehoming) and flock members for a very long time. But experiences tell us they really do.
I don’t know of there is a research...
Sorry for your loss.:hugs
I just make a guess what might cause her dead.
Maybe this. Her age in combination with her (over)weight and getting too much treats.
Maybe giving her a new home did not much good and added up to it. But living with other chickens as a species that thrives best in a...