Solution found (no thanks to Quickbooks): Quickbooks no longer matches deposits to payments, but to invoices. Do not book the payment transaction before importing the bank's deposit transaction. When the deposit transaction is imported, it should match to the invoice and then create a new payment transaction.

I learned this when a third deposit transaction was imported, one for which I had not yet processed the payment transaction. I saw that Quickbooks matched it to its invoice. I then deleted the other two payment transactions, the ones that were the subject of my original post; then Quickbooks did match the imported deposit transactions to their matching invoices and created new payment transactions for each.

This is actually preferable. It matches my workflow from three years ago, before we switched to Quickbooks.

But Quickbooks didn't provide any information that it was changing matching of deposit transactions from payments to invoices. No one from Quickbooks came here to explain that there had been a change. The one response from a Quickbooks employee said nothing about it.

This is just another one of those frustrating experiences that goes in the "find a replacement for Quickbooks" category.