Happy you got it sorted, but these are normally dual action rods, it should not have come undone?
Agreed, but from I what I just learnt about dual action TR's, the bottom of the rod is not fixed but just threads into a another "nut".
Theoretically the round rod of the assembly could be unscrewed and actually extracted from the neck. (not that I'd try it )

That said, I'd be dubious about nut & rod threads that have enough play in them that they would vibrate loose from transport.
I suppose once tensioned by the strings, it may not be an issue again.