Thanks for your reply.

When you've seen this before, did they get a 2020 1099-B with a sale date of 6/30/19? (obviously not those dates, but that type of scenario.)

By entering what is on the 1099-B, I generate an error saying that I cannot claim a 2019 stock sale in 2020.

I was able to learn that this was for a company that went private, and I did confirm that the transaction was fully settled in 2019.