It's not a "setting" -- not a Property you can switch off. It's core functionality that's existed for the 20 years I've been working with Maximo, that if the Location has one and only one Asset, Maximo automatically populates the Asset field with that Assetnum.
There have been other posts here in this forum and in the IBM community for Scripts that can be used to make it stop doing that. One such example:
MaximoDev - Disable automatic asset/location population in Work Order and Ticket applications (bportaluri.com)