This has all worked fine up to this morning.
Today, during a OneStep update, I received a message that Merrill Lynch needed re-authorization. I went through the process, and all went smoothly on Merrill's end. When I was returned to Quicken, I got a message that there were no accounts to match. I then triggered another re-authorization with the same result.
Going into account details, de-authorizing and then re-authorizing seems to have fixed it. After the Merrill end of re-authorization completed and I was returned to Quicken, the correct account appeared to be re-linked to my corresponding Quicken account.