If the router reboots, it cannot remember the correct mapping of the last connection, right? (The mapped port may not be the original source port)
That's why port preservation is the usual behavior.
(edited) Correct, this pickup of ports from the ACKs that the internal host keeps resending only works if the externally mapped port didn't change from the default (same as internal).
wouldn't it also work for connections established on another router that then failed or otherwise had traffic shift to it?