I have had the exact same issue as the original post with my HomePod Mini and Pandora account. I have gone through all the troubleshooting steps listed and I can get my Pandora account linked for up to 7 days before I have the same problem of it not being connected through the App. I have kept a log of when I make the connection in the Pandora app with my HomePod and when I use my account in other ways. No correlation.
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