FK constraint question

Can something explain why something like the following could happen?

INSERT INTO task_history (task_id, user_id, task_date)
SELECT task_id, user_id, task_date
FROM task
WHERE task_id IN (1, 2, 3);

ERROR: insert or update on table "task_history" violates foreign key constraint "fk_task_history_task_id"
Detail: Key (task_id)=(1) is not present in table "task".

Thanks,
Manfred

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