The answer seemed to be that the taxable IRA withdrawal for me (spouse) was from an Inherited IRA. For my husband (taxpayer), the taxable IRA withdrawal was an ordinary distribution. I finally figured it out by switching the box 7 code from 4 (death) to 7 (Normal distribution). It was the only difference I could determine about our 1099-Rs. So apparently even though the IRS considers the withdrawal from and Inherited IRA to be taxable to me, it doesn't consider the asset value of the Inherited IRA to be part of my basis, and thus Turbotax decided there was no reason to create an 8606-S. I hope Turbotax might add some verbiage to explain this nuance someday, as I spent hours and hours trying to figure it out.
Anyway, my problem is resolved.
