I was able to get this to work, both in 2020 and 2021. I have already filed my 2021 returns, so I don't think TurboTax will allow me to go back to figure out what I did. My 2021 Smart Worksheet does have both line "a" and line "b" filled out with the same value. I remember multiplying the "Exempt Interest Dividends" amount by the percent that belonged to Wisconsin. That product is what I entered for the Wisconsin amount. It showed up in the Smart Worksheet on line "a" and line "b". When I entered that value, I thought I did the same thing that you described. But there may have been some other thing I checked or filled out that I am not remembering. One thing you can double check is to make sure the sum of the Wisconsin amount and "all the rest" amount is equal to the original Exempt Interest Dividends figure.
