Thanks for the response, I feared it was the factory filter, known down here to pass dust on the diesel versions. K&N are not ideal for true off-road scenarios IMHO.
Sounds like another reason for me to follow up the new Donaldson setup (advertised for the diesels) to check compatibility with the URJ. Donaldson air cleaner for Diesel - https://forum.ih8mud.com/threads/donaldson-air-cleaner-for-diesel.1299380/
I believe the dusting box is only on diesel engines right? There are some nice metal airbox upgrades available down there too. Not sure if theres anything to worry about here, but I may run some oil with my finger around the seal on a new filter. Never seen any evidence of dust.