The original fitting seems to stay put OK, certainly for earlier models, due to the excessively strong threadlock (aka superglue) Yamaha use.
There seems to be a suspicion that later models (i.e. yours) may have an undersized threaded section on the shaft. It might be worth measuring diameter of the exposed tip to see if it's less than 17.8mm. This would be useful info for everyone since with 13,000-odd miles on it it sounds very likely yours is one that's never lost the nut and is in the same state as it left the factory.