Thanks for your input.... the transaction was fully settled in June of 2019, reported on a 1099-B at the end of 2019.

And now a year later, an extra few hundred dollars were reported on a 2020 1099-B with the same June, 2019 stock sale date.

I was able to confirm with my client that the transaction was fully settled mid-year in 2019. I'm fine reporting it, but I cannot enter the 1099-B as delivered without generating an error that says I cannot report a 2019 stock sale on the 2020 return.