The TaskPersistenceManager is holding onto the original value read from the database. Therefore, when the TaskCompletedHandler.handleCompletedTask (the private class inside SyncWSHumanTaskHandler) attempts to retrieve the task, it gets the original, not the updated and simply calls abortWorkItem(long).

In an effort to work around the issue, I modified TaskPersistenceManager by adding a method as such:

public void refreshEntity(Object entity)

I then modified the LocalTaskService.getTask(long) so it refreshes the entity by calling TaskPersistenceManager.refreshEntity() before retrieving it again.