(sobbing) If ever there was a year for this to happen, it's a doozy. The missed 1099 has $17k in dividends and $2.175M (yes) in gross proceeds. Yields large capital loss carryover.
Thank goodness HUGE refund on the originally filed return.

Strategy question: The refund on the original file was split between direct deposit refund and 2020 estimated tax. Is it cleanest to get the full refund or apply the full refund to 2020 estimated tax payment?

California works same way? Paper file writing Superseding in the return?