Ok, I finally found another question with the same issue. Looks like rounding entries to the nearest dollar makes it work.
Not sure why. I didn't have to do that last year. There were only two entries on the form, 1a and 1b, exactly the same number. Frustrating.
TT ought to give out partial discount vouchers for next year's purchase to those discovering, and especially you kind people helping to overcome, their bugs.
TT could round their vouchers to the nearest dollar of course.