This was a recent change made to the way that Battle.net works for users with multiple accounts. This information is now stored server-side, and no longer locally in your config.wtf file.
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That would explain why I couldn't find the realm name in the various .wtf files, where I thought I'd seen them listed before, a looong time ago; hadn't poked around in those files again until today.
Thanks for the clarification.
I have not been asked for the authenticator code since this change ;)
Maybe Lax can make something for this in the quicksetup ?
Ribbs.