Unfortunately, thisis due to a known bug that has since been fixed but has left some scenes that had duplicated template instances corrupted where the editor thinks they are templates but really they are just plain entity instances.
To fix the issue above, please read this thread Cannot find child entity of parent "Root" - Editor error, can't delete entities
We are due to release a migration tool next week to detect scenes that have these broken template instances and migrate them to a stable state.