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The reason for this is that persistence annotations only apply to primitive objects, and not collections like lists and maps. The latter can be changed in the local instance without automatically changing the distributed copy. Detaching, changing, and then reattaching such an object does change it in the persistent store.
Persistent annotations for functions that modify list structure are particularly problematic; the persistence factory doesn't know what to do with, e.g., list.add(thing). So, among other things, one must avoid using collections as persistent variables to avoid this problem.