I use Yakihonne on mobile and on desktop with my main npub and I have not had my Lightning address replaced. So it doesn't happen automatically. Maybe if you go through the process of creating a NWC wallet, the onboarding is assuming if you created a NWC wallet that you want to use it? I have only created NWC wallets on Yakihonne with new nsecs. So I cannot comment on creation with my npub but I will go and test that right now.