Can you check if the task has been rejected by all assignees? And is it possible that the task can somehow be seen again on the system or gone for everyone else.
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As far as I know, a task can always be seen in the Process Instance (by a user with permissions sufficient to see the process instance, etc), and then manually reassigned from there to basically any user. I don't think there's any systematic way to check what user(s) have rejected a task assignment, though.
AFAIK a task cannot be rejected by the last assigned person.