Tracing the "Data Source" backwards in TurboTax leads to the "Qualified Business Income Deduction Summary" form, Line 9, "Total REIT Dividends." TurboTax says that the "Data Source" for this line is the Schedule K-1, but there isn't any K-1 in the return.
Turns out the true data source is the aggregate of the Box 5 entries in the 1099-DIV's, which reports "Section 199A dividends." TurboTax does not indicate the flow of this data in any Help pop-up.