Well, there's no way that this is Schedule C self-employment income. It still sounds like interest to me, unless there's some other component of the coupon payment that I don't understand. If the brokerage sent you the equivalent of a 1099-MISC, with box 3 filled in for "other" income, I'd report it as a 1099-MISC, but continue with TurboTax to say that it's interest on a note, that it doesn't involve work like your main job, that you got in 2018 (only check 2018) and that it does not involve an intent to make money. I know that sounds counter-intuitive, but that's how you get the income reported as Other income, not self-employment income. If this isn't on a 1099 equivalent (brokerage statements can be quite involved). I'd just report it as interest income. And I'd surely ask the brokerage the reason for the change.
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