I hit this as well, though going the opposite way, where my income went up slightly, but TurboTax was saying $0 owed. Using the desktop version of the app, I eventually chased this down. There's an interview question where you put in previous tax paid or refunded. At least for the tax paid box, TurboTax was taking in the number, but then not filling in the corresponding value on the income tax return correctly. (For 2019, this is NC Form D-400, Line 22.)
The workaround I found was to modify the tax return directly. If you go to review the amended return, you will eventually be shown a screen with the filled-out D-400. I found Line 22 was being filled with a value that equated to the tax I previously paid plus the tax I owed, resulting in the tax due of $0. When I changed Line 22 to have just the amount I previously paid and saved the form, the value for Tax Due was recalculated, and was correct. (I would guess the same would work for Line 24, "Previous refunds.")
I spent a couple hours on the phone with TurboTax support about this, but it was not clear to me at the end of all that whether they would kick off an investigation into this apparent bug.
