After much investigation, I have concluded that, for my return, this bug did not change the amount that TurboTax said that I owed to Missouri. In particular,
The error occurred when I had 1099-DIVs for “both of us” and reported income from US government obligations in the interview. On my return, TurboTax subtracted the amount entered in the interview from the taxpayer amount on MO-A line 8 and added twice the amount entered to the spouse amount. Since the taxpayer and spouse amounts are added together later, the total subtraction didn’t change.
I still consider this a bug since the amounts on MO-A line 8 are not allocated as expected between the taxpayer and spouse. I do not know if the error could result in an incorrect calculation of tax owed for a different return. During the investigation, I did see other unexpected behavior with respect to the computation of MO-A line 8 but I haven't figured out what TurboTax was doing for those cases.
