I believe found the issue. Here are the detail for anyone else in the same situation:
In previous years, for which we had a DCFSA, we both had earned income. In 2024 one of us was not employed for at all during the tax year, and had no individual earned income. And while dependent care expenses allowing "you to work or look for work" is normally eligable as a qualified DCFSA expense, if one partner has zero earned income for that year due to not having a job, dependent expenses enabling job search are not deductable at all (per IRS Publication 503 (2024)).
In TurboTax defense, it does mention this in Deductions & Credits > Dependent Care Credit. It seems to have detected a single W-2 and made the determination and moved on. It was confusing, and would have been nice if it were called out a little more clearly, but the logic isn't off.
