Why might I have a 2023 tax liability/penalty when calculators confirm my withholding is set appropriately and I paid outstanding taxes in full?

If you're preparing your 2024 return, the underpayment penalty you see is for 2024 - not 2023. Your 2023 tax liability is just used to calculate whether you have a penalty for 2024.

For federal tax, if you have a balance due of $1,000 or more you may be subject to an underpayment penalty. To avoid this penalty, you will have had to pay at least 90% of your current year (2024) tax liability or 100%* of your prior year (2023) penalty. If, after entering your 2023 liability, TurboTax is calculating the penalty, you didn't pay the minimum required payment during 2024.

*110% if your income was $150,000 or more